Hi Kerim,
Thank you for sharing these details.
If you'll check the settings of the relationship field "Verbind hiermee je profiel aan de post", you'll see the option "Field default value".
Screenshot: hidden link
If you'll add the ID of the current user's "Trainer" post, it will be automatically selected/filled in the form and the visitor won't have to type it.
( as needed you can also hide this field from the front-end so that the pre-filled field is not visible in the form at all )
To get the ID of the current user's "Trainer" post, you can create a new post view, that is set to show trainer posts but with an author filter ( https://toolset.com/documentation/user-guides/filtering-views-query-by-author/ ), so that it only brings the post where the author is the current user.
In that new view's loop item content template, you'll only need the "[wpv-post-id]" shortcode, so that it only returns the ID of that post.
https://toolset.com/documentation/user-guides/views-shortcodes/#wpv-post-id
To remove any extra spaces from this view's output, you'll need to take a couple of more steps.
There will be a checkbox for "Disable the wrapping DIV around the View" option, just above the "Output Editor" section on the view's edit screen.
( screenshot: hidden link )
Please check that option and the view's wrapping DIV container will be removed.
Also remove the shortcode [wpv-filter-meta-html] from it's "Output Editor" section and also remove all line breaks and empty spaces from the content in "Loop Editor" section, making everything in the same line.
Once this view is ready, you can place its shortcode in that "Field default value" option.
https://toolset.com/documentation/user-guides/views-shortcodes/#wpv-view
Important note: You'll also find useful information on linking users with their proxy trainer post type, in this guide:
https://toolset.com/documentation/post-relationships/how-to-create-custom-searches-and-relationships-for-users/
I hope this helps and please let me know if you need any further assistance around this.
regards,
Waqar