Add project to Portfolio via CA or WF

I would like to add a project to a portfolio either by a WF or CA.  

To test this via a CA I have a variable that is a reference to object - portfolio, which allows me to select the portfolio  want to add the project to.

The Actions are to Update Field and this is where its going wrong.  I cant figure out which action is required to update the $Portfolios field in the project.

Any help would be much appreciated.

Thanks