OK so here:
https://fluentforms.com/docs/fluenform_before_submission_confirmation/
I found an action that FF folks recommend to use if I want to execute any custom functionality.
I'm trying to adjust the code you kindly provided and I just don't know how it's suppose to looks like.
I also tried to exactly follow your loom tutorial (with use of MB forms) and it didnt work as well. Only difference I made was I used build-in relationship builder instead of coding it with codeblock.
Sorry for being noob ๐