<form id="form1" method="post" action="https://webleads.abinitiosoftware.co.uk/api/LeadDetails"
enctype="application/x-www-form-urlencoded">
 <input type="hidden" value="value1" name="AB_CUSTID" /> //mandatory
 <input type="hidden" value="value2" name="AB_PWORD" /> //mandatory
 <input type="hidden" value="https://www.abinitiosoftware.co.uk" name="URLRedirect" /> //optional
 <input type="hidden" value="1" name="TEST" /> //optional
 <input type="text" name="FIRSTSURNAME" />
 <input type="text" name="EMAIL" />
 <input type="submit" />
</form> 

<%
test = 0 //1 = test mode, 0 = live
ab_custid = yourCustomerID
pword = "yourpassword"
firstsurname = "value read from your data"
firsttitle = "value read from your data"
houseno = "value read from your data"
street = "value read from your data"
town = "value read from your data"
pcode = "value read from your data"
leaddate = "" //blank will use current date, else if leaddate entered, must be in
"DD/MM/YYYY" format
DestURL = "http://webleads.abinitiosoftware.co.uk/api/LeadDetails"
Set xmlhttp = Server.CreateObject("msxml2.ServerXMLHTTP")
xmlhttp.Open "POST", DestURL
xmlhttp.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
postdata = "TEST=" & test & "&AB_CUSTID=" & ab_custid & "&AB_PWORD=" & pword &
"&FIRSTSURNAME=" & firstsurname & "&FIRSTTITLE=" & firsttitle & "&HOUSENO=" & houseno &
"&STREET=" & street & "&TOWN=" & town & "&PCODE=" & pcode & "&LEADDATE=" & leaddate
xmlhttp.Send postdata
Response.Write xmlhttp.ResponseText
%>

$url = 'https://webleads.abinitiosoftware.co.uk/api/LeadDetails';
$houseno = value from your data;
$firstsurname = value from your data;
$pcode = value from your data;
$custID = your Customer ID;
$pw = your password;
$data = array('AB_CUSTID' => $custID, 'AB_PWORD' => $pw, 'FIRSTSURNAME' => $firstsurname, 'HOUSENO' => $houseno, 'PCODE' => $pcode);

// use key 'http' even if you send the request to https://...
$options = array(
    'http' => array(
        'header'  => "Content-type: application/x-www-form-urlencoded\r\n",
        'method'  => 'POST',
        'content' => http_build_query($data)
    )
);
$context  = stream_context_create($options);
$result = file_get_contents($url, false, $context);
if ($result === FALSE) { /* Handle error */ }

//var_dump($result);

/**
* POST Call to AdminBase API from CF7 Submit
*/
add_action( 'wpcf7_before_send_mail', 'wpcf7_api_function' );

function wpcf7_api_function( $contact_form ) {
    $title = $contact_form->title;
    $submission = WPCF7_Submission::get_instance(); 
    if ( $submission ) {
        $posted_data = $submission->get_posted_data();
    }      
   if ( 'Contact Form' == $title ) {//The title will be form specific!!
     $name  = strtolower($posted_data['your-name']);//YOUR-NAME will be form specific!!
     $email = strtolower($posted_data['your-email']);//YOUR-EMAIL will be form specific!!
     $phone = strtolower($posted_data['your-telephone']);//YOUR-TELEPHONE will be form specific!!
     $notes = strtolower($posted_data['your-message']);//YOUR-MESSAGE will be form specific!!
     $leadsource = "WEB";
     $ab_custid = "CUSTOMER SPECIFIC";//AB_CUSTID will be customer specific!!
     $ab_pword = "CUSTOMER SPECIFIC";//AB_PWORD will be customer specific!!
 
     $url = 'https://webleads.abinitiosoftware.co.uk/api/LeadDetails';
 
     $data = array(
        'AB_CUSTID'     => $ab_custid,
        'AB_PWORD'      => $ab_pword,
        'FIRSTSURNAME'  => $name,
        'EMAIL'         => $email,
        'HOMETEL'       => $phone,
        'NOTES'         => $notes,
        'LEADSOURCE'    => $leadsource,
     );
     
     // use key 'http' even if you send the request to https://...
     $options = array(
        'http' => array(
            'header'  => "Content-type: application/x-www-form-urlencoded\r\n",
            'method'  => 'POST',
            'content' => http_build_query($data)
        )
    );

    $context  = stream_context_create($options);
    $result = file_get_contents($url, false, $context);
    if ($result === FALSE) { /* Handle error */ }
    }
} 

{
    "token": "8548d5e0-9d47-4d08-b9ec-2e0d8689c712"
}

var handleSubmit = (event) => {
  //event.preventDefault();
  
  const form = document.getElementById("form");
  const inputFile = document.getElementById("file");
  const inputNote = document.getElementById("note");
  var formData = new FormData();
  
  //set values of formdata - these are the fields of the form i.e.
  formData.append("NOTES", inputNote.value);
  // ...
  formData.append("TOKENREQ", 1);
  formData.append("AB_CUSTID", "Your credentials");
  formData.append("AB_PWORD", "Your credentials"); 

  //send form data
  fetch("https://webleads.abinitiosoftware.co.uk/api/LeadDetails", {
    method: "post",
    body: formData,
  }).then((data) => { //if successful send file(s)
    for (const file of inputFile.files) {
      formData = new FormData();
      formData.append("file", file);
      formData.append("token", data.token);
      formData.append("description", file.name);
      formData.append("filename", file.name);

      fetch("https://webleads.abinitiosoftware.co.uk/api/Upload", {
        method: "post",
        body: formData,
      }).catch((error) => ("Something went wrong!", error));
    }
  }).catch((error) => ("Something went wrong!", error));
};

<form id="form">
 <input type="text" id="note" />
 ...
 <input type="file" id="file" multiple />
 <button onclick="handleSubmit()">Submit</button>
</form>