Messing up relationships between entities

Dear reader,

I’m running around in circles now, so maybe someone could give me a hand or direction?

Situation:

On entity Systemuser I created a synchronous workflow that runs on request only. All it does (should do) is copy a few fields from another record in another custom entity (let’s call it B). The relationship between Systemuser and B is 1:1.

So far, the fields that are copied, remain blank. This makes me suspect that the link/relationship between Systemuser and B isn’t quite as it should be. Yet I do get to select B in the dropdown on the workflow, probably since I created a one-to-many relationship from B to Systemuser. But obviously I’m making somekind of mistake.

How to setup the field/entity relationships correctly?

Thanks for thinking along with me.

Regards,

Jeroen