Are you trying to avoid having the user do this via the Okta UI? Once a user is created, they can enroll in MFA factors themselves by going the the Settings menu from the end-user dashboard: https://oktaDomain/enduser/settings
If you want to do this via your own UI, you will not be able to do so via the widget unless the user meets the criteria for MFA enrollment, based on your policies.
You may want to look into making these API calls directly using our Factors API so that you can enroll users in enabled factors.