Adding MFA factor

I need to add MFA factors to a user. While the process laid out in this documentation Authenticate Using the Resource Owner Password Flow with MFA works fine it requires the Resource Owner Password Flow, which requires the user input their username and password. At this point in our app the user is already authenticated via MFA. I do not want them to have to input their username and password to get the MFA token.

How can I add a new MFA factor (/mfa/associate or any other means) without requiring the user input their username and password?