You could check the name of the aircraft by looking at the FSP Company Manager or looking at the aircraft.cfg file of the B738, its name is v4EHM-B738
Go to your FSPassengers folder in your main FS folder, look for payload editor under the payload models folder. Open up Payload Editor, load the ini file for your B738 payload model, jump straight to Step 4 on the editor and save a seperate copy with the name of the B738, v4EHM-B738. Then just load up FS and you're good to go!