Populate User Name from Sharepoint Server to Certificate

Hello:

I work for a govt contractor and we are loading our CBT on a Sharepoint server. The server guys have created a hidden field within my HTML file called

Input html ID: CBTUserName

So when the user accesses the CBT from the Sharepoint Server (which is NOT an LMS) their User Name populate.

Now I need to get that user name to populate in the Certificate so that users are NOT required to type their name. Our customer (military) does NOT want to ask users to type in their name they want it automatically populated from accessing the CBT through the server.

HELP- I am a beginner and don't know where to start or what to do.

Thank you!