When you have to manually install the Azure Site Recovery agent you must provide the Passphrase. In this blogpost I describe how you can find the Passphrase.
Go to your ASR installation directory (for example \\Program Files (x86)\Microsoft Azure Site Recovery\Agent.
Look for the genpassphrase.exe file and start it using administrator credentials.
After executing you can find a file named connection.passphrase. Open this file using your favourite editor and you have found your passphrase. Good luck!