From an entry in 2018:
v5.1.3 has been released now with the UI fix to show the PFX password selection options.
The option is enabled under Settings > UI Settings > Custom PFX/Private Key password then relaunch UI and choose PFX password credentials under Certify > Advanced > Signing & Security.
[I am using the Community Edition] I can navigate to that. However, the dropdown only shows ‘NO PASSWORD’. I am unable to add a password and therefore, unable to use the PFX in my Server Essentials 2016 Anywhere Access setup.
Is there a default password for .pfx files?