HTMLFormElement

const f = document.createElement("form"); // Create a form
document.body.appendChild(f); // Add it to the document body
f.action = "/cgi-bin/some.cgi"; // Add action and method attributes
f.method = "POST";
f.submit(); // Call the form's submit() method

<form name="formA" action="/cgi-bin/test" method="post">
  <p>Press "Info" for form details, or "Set" to change those details.</p>
  <p>
    <button type="button" id="info">Info</button>
    <button type="button" id="set-info">Set</button>
    <button type="reset">Reset</button>
  </p>

  <textarea id="form-info" rows="15" cols="20"></textarea>
</form>

document.getElementById("info").addEventListener("click", () => {
  // Get a reference to the form via its name
  const f = document.forms["formA"];
  // The form properties we're interested in
  const properties = [
    "elements",
    "length",
    "name",
    "charset",
    "action",
    "acceptCharset",
    "action",
    "enctype",
    "method",
    "target",
  ];
  // Iterate over the properties, turning them into a string that we can display to the user
  const info = properties
    .map((property) => `${property}: ${f[property]}`)
    .join("\n");

  // Set the form's <textarea> to display the form's properties
  document.forms["formA"].elements["form-info"].value = info; // document.forms["formA"]['form-info'].value would also work
});

document.getElementById("set-info").addEventListener("click", (e) => {
  // Get a reference to the form via the event target
  // e.target is the button, and .form is the form it belongs to
  const f = e.target.form;
  // Argument should be a form element reference.
  f.action = "a-different-url.cgi";
  f.name = "a-different-name";
});

<form action="test.php" target="_blank">
  <p>
    <label>First name: <input type="text" name="first-name" /></label>
  </p>
  <p>
    <label>Last name: <input type="text" name="last-name" /></label>
  </p>
  <p>
    <label><input type="password" name="pwd" /></label>
  </p>

  <fieldset>
    <legend>Pet preference</legend>

    <p>
      <label><input type="radio" name="pet" value="cat" /> Cat</label>
    </p>
    <p>
      <label><input type="radio" name="pet" value="dog" /> Dog</label>
    </p>
  </fieldset>

  <fieldset>
    <legend>Owned vehicles</legend>

    <p>
      <label
        ><input type="checkbox" name="vehicle" value="Bike" />I have a
        bike</label
      >
    </p>
    <p>
      <label
        ><input type="checkbox" name="vehicle" value="Car" />I have a car</label
      >
    </p>
  </fieldset>

  <p><button>Submit</button></p>
</form>