1. Go to wp-admin->lms settings->registration forms.
Create a new registration short code.
2. Copy the custom registration short code created.
3. Create a new page and paste that short code there.
4. Go to wp-admin->settings->buddypress->pages->registration ->set it to none.
5. Go to wp-amdin ->WPLMS ->buddypress-> Enable both ajax login and registration.
6. Go to wp-amdin ->WPLMS ->buddypress-> Custom registration form ->set it to the page created by you which has the registration form short code.
ALSO CONFIRM PASSWORD field is not avaiable in our custom registration forms as of now.
Now you will be able to register from within the login popup menu using ajax.