The Activate app allows two ways to sign in to it: a) password or b) pin. 

Initially, you will need to sign in with your password but once in, you can generate a pin by clicking on the side menu in the app.

Select 'Generate Pin'. 

Create a pin for your account. 

The next time you log in, you'll be presented with a screen with the option to login with password or pin. Click on 'Use my Pin' link to be taken to a new login page where you can login with your selected pin. 

If you have forgotten your pin, you can use the 'Forgotten your pin?' link to have it reset.