pin for login

What pin? If you are connected on A1 mobile network you will be automatically log in with your number, so I guess you are not in Croatia?

I don't know if you can use My A1 app out of Croatia, you can try registrate yourself in app or web My A1 

How to start?
 1. Download and install the My A1 app
 2. Switch to mobile internet if you are on wi-fi
 3. Log in automatically with your number
 4. If you are not a mobile user, enter your username and password
 5. Browse our introductory screens and play around with the app
 
 When you are connected to an A1 network, using the My A1 application does not consume data traffic.