Make sure any other filament is unloaded, if you have been using the AMS it does this automatically.
Use the Bambu Studio or Bambu Handy app Device tab and select the external spool.
Press the load button.
This will begin heating the hot end.
Make sure you have feed filament through the back of the printer manually and right up to the hot end.
The hot end will get to temperature and you should apply gentle force to the filament so it gets into the hot end. The printer should take over pulling the filament in. You may be asked to confirm filament was extruded.
Once complete, you can print.
As you are using the external spool, you will need to use the unload option to do the process in reverse once you no longer need the external spool. If you do not, the AMS will not be usable until you have.