Comparing two fields with a If condition in a process workflow

Hello,

I want to create to create a process background workflow, where I want to have a simple If condition,

It should simply compare the Primary Contact field from Account with the Full Name field from Contacts. But I can not select a field which I want to compare with, I can only select a specific contact via a lookup search.

To make the comparison vice-versa also does not work, compare the Full Name field from Contact equals with... only allows me to compare with a value which I manually enter, see screenshot below.

Any ideas?

Thanks!