This will only work if you’re using the Classic SHM app, the new SHM app cannot be controlled via external apps as yet. If you have access to RBoy Apps, check out the LUM app which allows you to create custom actions when you use your lock keypad (e.g. lock or unlock, manually or with a code etc).
If you’re looking to use a keypad (a standalone ZigBee keypad for example, which are pretty cheap), then you can use LUM above to do the same and synchronize arming/disarming SHM with the keypad and the lock at the same time - so you have a choice of how to arm/disarm and even check the SHM status (the keypad lights will show the current SHM status).
Again all this only currently works with the Classic app SHM. I know you said you wanted to use the new app SHM, is there a specific reason for using that when the Classic app has so many more options available?