Hello @Mero
Please register on SwissPass (www.swisspass.ch) and once you have your login (email + password), you can log in in the SBB Mobile App as well.
You do not need a customer number for this.
When travelling with the Swiss Travel Pass, just show the PDF e-mail you received.
If you have any furhter question, please call our collagues at the SBB Contact Center (0041 848 44 66 88, CHF 0.08/min), they are available 24/7.
I wish you a nice weekend.
EleonoraS