The entire Swiss Travel System ticket range - and therefore also the Swiss Travel Pass Flex - is available as an e-ticket. When you purchase a Flex Pass, you will receive a receipt and detailed instructions on how to activate the individual travel days.
Just like the physical pass, you can freely choose the travel days (3, 4, 8 and 15 days) within a month. To do this, you activate one travel day at a time before you travel on activateyourpass.com
In this case, integration in the mobile app is not possible - sorry for the misinformation.
For more information, call the SBB Contact Center on +41 848 44 66 88 (CHF 0.08/min.).