Workflow - Update user field on a related module


i'm using Enterprise on-prem v12.0.4. I have projects that can be linked to one or more Opportunities.
I want to have two fields updated in the Project any time an opportunity is added or modified.
I have created two fields in Projects: a Datetime field (Last Opportunities update), and one Relate field linked to users module (Last Opportunity updated by).

I'm trying to create a workflow for this scope, but, while updating the date field works, i cannot find a way to update the user field. I mean i cannot even select it. I would also consider the option to update the standard Date Modified/By field in the project instead, if this made things easier, but it is not visible in workflow selection as well.

Can you please suggest any solution?