Need username to = first_name+last_name

I have a registration page created using the Ultimate membership plugin, I don't want people to have to enter a username for registration. I tried hiding the username field and using some code, changing the display name to first name & last name.

However, since WordPress requires a username by default, it uses the user's email address as the username instead. For example this account (https://****

I am wanting to automatically make the username = first_name-last_name . rand()

We thought just making the username field somehow a hidden field on the registration form, and then perhaps with some Javascript we could automatically fill in the user_login field with the $_POST data for first_name & last_name with a random number at the end when they fill in the form.