If the employee you are getting the pass from is a United or United Express employee, then you can only travel on United or United Express flights. (no Lufthansa or other Star Alliance airlines). Only the employee, their spouse, and children have the ability to fly other airlines under employee travel.

You can list for any flights you want. The employee is charged per each segment that you are boarded on.

United service to BKK ends April 9, so you will have to travel before then in order to fly on a buddy pass. After that you will have to buy a ticket to somewhere that UA flies to (I recommend TPE since that will be a new flight that should not be that full).