In GUI -> Administrator -> Licensing -> Click here -> Continue -> Click on Create Sophos ID -> Enter Email ID and Click on Send Verification Code -> Enter Verification Code and Click on Verify Code -> Click Next.
Once Account is Created go Back and Click on Sign In
Again go to Sign In and Click on Sign In and Login with Created Credentials -> Verify and Click Continue -> Click on Confirm Registration -> Click on Initiate Synchronization -> Click on Add Licenses Key -> Enter the Licenses Key (You might have received at the time of purchase) and Click on Verify Key -> Confirm and Click Continue.
Click on Administration -> Licensing -> Click on Synchronize and all the Licenses will show
Note:- To add Additional Licenses Click on Activate Subscription and Add Licenses Key and then Click on Synchronize.