How do you specify which line to use to dial a contact?

I have an external_line defined to a different Asterisk server and need to configure a button to dial a contact on that server using the external_line’s account info.

I can specify the action’s dial=“sip:1000@otherpbx” but the INVITE is sent using the primary line’s credentials so it fails.

Can’t seem to find a way to specify which line’s credentials to use.