How to add the domain to the Link field?

Question

I activated the Link module that is part of Drupal core. I want to add on the user profiles fields with links to social pages and limit the domain used in the link.

How can I add such restriction to the link field?

I also tried the Social Media Links Block and Field module, but it uses a third-party library for the icons and it does not suit me.

I would like to make the link form elements appear like in the following screenshot.

Answer 1

I would do the following:

1. Implement hook_field_widget_single_element_form_alter(), and add an #element_prefix key in front of the URL field with the domain. This will handle the display.

2. Create a custom validation constraint that limits the URL element of the Link field, and add it to the field on the user (using method #2 on the linked page).