If you want members to be able to create a custom username when they're signing up, first you have to make sure you have MemberMouse configured to allow members to change their username. 

Follow the steps below to confirm that members can change their username:

  1. From the MemberMouse menu go to General Settings, and then click on the Other Settings tab.

  2. Scroll down to the WordPress User Options section.

  3. If you check the box, members will be able to change their username at signup and also from the My Account page.

  4. Click the  button to save your settings.

Once this has been confirmed, now you have to include the following SmartTag in your checkout form(s):

[MM_Form_Field name='username']

In order for this to work correctly make sure that you place the SmartTag between the [MM_Form type='checkout'] and [/MM_Form] SmartTags. After this SmartTag has been included in the checkout form MemberMouse will require that this field be filled out before allowing the form to be submitted.