<form method="post" action="<https://SuiteletHostedOnFileCabinet.js>">
    <table>
        <tbody><tr>
            <td id="ttext">Test Text</td>
            <td><input id="input1" placeholder="Anything" name="Input1" required="" type="text"></td>
        </tr>
    </tbody></table>
    <br>
    <button type="submit" class="btn" formmethod="post">Test Form</button>
        <script>
            //testing if the Suitelet value passed through
            let queryString = window.location.search;
            let urlParams = new URLSearchParams(queryString);
            let id = urlParams.get('recid');
            document.getElementById('input1').innerHTML = id;
        </script>
</form>

/**
  * @NApiVersion 2.1
  * @NScriptType Suitelet
*/
define(['N/https', 'N/log'], function callbackFunction(https, log) {
  
  function getFunction(context) {

//record ID was passed from a Client Script to the Suitelet
    var id = parseInt(context.request.parameters.custom_id,10);
    let headerObj = { name: 'recid', value: id };
    var contentRequest = https.get({ url: "<https://HTML_hosted_on_File_Cabinet.html>", headers: headerObj });
    var contentDocument = contentRequest.body;
    context.response.write(contentDocument);
  }

  function postFunction(context) {

    let test = Object.keys(context);
    log.debug({ title:"Testing variable passthrough", details: test })
    context.response.write(test);
  }

  function onRequestFxn(context) {

    if (context.request.method === "GET") { getFunction(context) }
    else { postFunction(context) }
  }
  return { onRequest: onRequestFxn };
});