Open Browser on Login
To configure an action like Open Browser on Login follow these steps: 


Go to System Preferences->Accounts and select your user account in the left column
Then Select the Login Items tab on the right. There you see a list of applications and files that you can direct your Mac to open at login) 
Just drag your .webloc file(s) into the Login Items list


And the next time you log in, Your Mac will open automatically the web location (.webloc) you put in Login Items tab (with your default browser)



Here more detailed explanation about how add .webloc files on login startup:  https://www.macworld.com/article/1156341/open_websites_login_startup.html