Form API
Drupal has an API for creating and modifying forms so all modules can specify and customize them in the same way.
Form Alter Hook
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Articles
Set form page title
It's very easy to set the page title for your form. In your buildForm() or form alter function, set the #title property to...
Commerce Entity Select
Drupal Commerce has a field widget, Commerce Entity Select that does some interesting things with the field and form API.
Source...
Resources
Render custom entity form modes programatically in Drupal 8
How to render an entity form.
form.api.php
Form API code documentation.
Form API
Form API documentation.