APM branch rules based on select box

Hello,

Can somebody give me a hand on very simple matter (I'm new to F5).

Designing my APM policy, got Login Page element where I've introduced select box field and called it realm. Want to branch rule from it.

So for example if user select from list realm primary I want to follow a different route than when he will select branch secondary. What I've tried:

expr { [mcget {session.logon.last.realm] == "primary" }

However this does not work, can anyone advise how to do this? Basically, the problem I'm trying to address is I've got 1 portal and 5 different AD so the user needs to select what realm he will go against.

Thanks

D