Like the virtual F5, you’ll initially need to SSH to the virtual appliance and change admin password via CLI:

$ ssh -i ~/.ssh/mykey.pem admin@10.10.10.89

admin@PA-VM> configure
Entering configuration mode
[edit] 
admin@PA-VM# set mgt-config users admin password
Enter password : 
Confirm password :

[edit] 
admin@PA-VM# commit

Commit job 2 is in progress. Use Ctrl+C to return to command prompt

..99%........100%
Configuration committed successfully

Then go back to WebGUI, login as admin, and go from there.