Do you have already created an own CUSTOM LOGIN ?
You perhaps first should do that → creating your own custom LOGIN/REGISTRATION-FORM.
You need an example? Here it is… (2-different login-forms which can be designed by your own, like shown in the following example)…
https://russian-dima.wixsite.com/login-system
- Login first (create an own account, or use an existing/predefined Login-Account.
- After Login you will be able to navigate to —> SETUP.
- Open General-Setup.
- You will see this setup screen …
5) Click on Edit.
6) Enter PASSWORD: 12345
7) Change to —> “SIMPLE”
8) Save new SETUP.
9) Go back to Login.
10) Navigate to → “Sign-Up” (REGISTRATION)
11) Take a look what you got.
- Do the same steps again and set SETUP back to → “MAIN”
- Navigate again to REGISTRATION, take a look again, what happened?
All you can see can be done by own custom generated CODE.
You want to learn more about “how to create such a login”?
-Read my post, or click the follow button (you won’t miss informations in future anymore) 
BTW: In my example i do not use a LIGHTBOX, but it could also be a LIGHTBOX, if it is wanted.