Change Set ticket to Pending and define Service

[zell]

Hello!
I need to change ticket to pending state and set ticket's service (ticket received via email).
So, i push "pending" and trying to set service...

ticket pending.png

But it's locked and i cannot change it.

ticket no.png

I think it's can be done via change otrs's configuration settings, but cannot find what i need to change.

[crythias]

Services <-> Customer, Directly assigned to customer or default services on the left.