Tell us what you are trying to do?
I'm trying to populate field attached to relationship with post title (ID as value of each option)
Is there any documentation that you are following?
No. I want to add a reference field in the relationship fileds, but at the moment it is impossible
Is there a similar example that we can see?
https://toolset.com/forums/topic/populate-select-field-with-list-of-posts-in-a-custom-post-type/
https://toolset.com/forums/topic/populate-select-field-with-all-the-posts-that-are-published/
https://toolset.com/forums/topic/custom-field-type-that-list-posts-in-a-custom-post-type-or-custom-taxonomy/
I managed to display a field using wpt_field_options filter. But I would like to add select 2 and AJAX, as it is already implemented in the Types when we select related posts.
What do you recommend?