How to use drupal_get_form outside of a menu
As I understand it, you want to link to a page that displays a form and accepts certain parameters that will influence the form.


  2) I am also getting a message that says drupal_get_form is an unknown function.


That is because drupal_get_form() is not known when Drupal hasn't been formally loaded (bootstrapped). When you call PaymentMethodEditDelete.php directly, then Drupal doesn't know about it, and PHP doesn't know about any Drupal API functions. In order to have all Drupal functionality available, the point of entry must be the index.php in the webroot and not a custom script somewhere.

In order to achieve what I think you want, you need to define a so called menu callback. This is simply a url that is associated to a callback function. You can find more on the menu system in the docs to hook_menu().

In order to use this menu callback with drupal_get_form you would define the menu item like this:

function mymodulde_menu() {
  $items = array();
  ...
  $items['my/custum/url/to/form'] = array(
    'title' =&gt; 'My form title',
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('mymodule_custom_form_id'),
  );
  ...
  return $items;
}


This is the bare minimum to define a page that will render as a form.

Now in order to handle arguments, you can either pass them in as ordinary $_GET parameters that you can access normally inside your form builder function.

Or you alter the newly defined menu so that it will accept additional arguments and passes them into the form builder, as arguments to drupal_get_form (see Wildcards in Paths). Also have a look at this question for more information: https://drupal.org/node/2054083.

Some more reading I can recommend:


What is the basic concept behind hooks?
http://befused.com/drupal/page-request