I recently found myself in a unique position where I needed to modify a form action to be an absolute URL in Drupal.
The quick backstory is I am using Cloudflare to serve a subdomain as a subdirectory. The subdomain is Drupal, and the main website is WordPress. Therefore Drupal is getting served on certain paths on my WordPress website. The problem is the Cloudflare proxy setup only accepts GET requests.
The form I am trying to modify is built with Webform.
Note: You may embed the Webform module using the share feature, although this wouldn't suffice for my use case as I needed to tie the submission to the node it was submitted on.
I needed a way to send POST requests to my Drupal URL, which led me needing to modify the form action to use the same path but add the domain to it (pointing to the original URL).
I first tried modifying the form action in hook_form_alter but was unsuccessful. While it modifies the action, it messes something up in the form and doesn't submit. The forms use some placeholder in the action at that stage in the rendering process, and changing it disrupts its flow.
Here is how you modify a Webform's action in Drupal:
Implement this, and the form action will be an absolute URL pointing to the origin (or change the value to match your use case).
The form submits!
You need to set up submission handling and determine where it redirects to after submission.
Happy form building and modifying form actions in Drupal!