Pitmac1 you just have to make it visible for as long as it takes to edit your process and tick the checkbox on the action instance data window for your “Set SelfService Flag”. Once you’ve done that and saved your process you can hide it on the window again.
What we do for this kind of thing is to have a "Process Window", set as Console Default. We only use console for design work. We use Webdesk / Workspace for Analysts / End users.
Then on this window we have all the attributes we need to update via a process like you want to do here.
Yes for sure that works by having an “admin/design” window and I do that myself, but I was just keeping the answer to the fundamentals; ie the field is only needed at design time and doesn't impact how the process works at runtime if it is not showing.