The same way you make any extension select lines when going off hook - Auto Line Selection.
From Ext. 10 or 11
Feature 00
Left Intercom button Twice
Right Intercom button Once
Dial the extension number of the Transtalk phone
Dial **
Press the line buttons in the order you want them to be selected
Dial ** again
Feature 00
That being said, I prefer to set the Transtalks to pick Intercom first, then the lines, so if I turn it on for the purpose of selecting a particular line on hold, it doesn't grab a free line until I hit the line button. In this senario, you can turn it on and press 9 for an available outside line for making a call.