If you have had ANY dealings/applications/visas before you will have a UCI (unique client identifier).
Once you have one..it's yours for any immigration process.
If you have not and have no UCI it will appear in your AOR (acknowledgement of receipt). This will come to your (the PAs) email address probably around 6-8 after your application is received.
If you have no UCI then this should be left blank on all forms.