How to query a Metabox post field

I’m trying to use a meta query in order to create a button (with specific styles) linking a testimony to its project, but am painfully failing.

In order to do this I’ve added a custom post field to the testimonies, which allows me to select the project directly. And then inside the testimony I tried creating a meta query with posts where {mb_testimony_project} equals {post_id}, I tried title, id, brackets, no brackets, none of it is right.

I also tried just putting the {mb_testimony_project}, aka the post field itself, in a simple link, which does function, but then I can’t use the styling from the project for the button.

Does anyone know how to properly query the post inside a metabox post field, so I can access its custom fields?