You can create a normal registered Login-Account with —> Email (adding it a nickname + password) and storing it for example in the Wix-Secrets-Manager.
When a user tries to use the login to log-in to your site, you run a query on backend and compare the entered nickname and pw with the one stored in your Secrets-Manager. This should do the trick (i think). 
Or you take a look here, onto a similar example…
-Navigate to “General-Setup”.
-Change “Active-Registration-Form” to → “SIMPLE” (this will switch the type of REGISTRATION-FUNCTION).
EDIT —> PW: 12345 —> change to SIMPLE → SAVE
-Now you can navigate to the REGISTRATION.
There you will find a similar example, which will create a new LOGIN without any e-mail. An upgraded version will follow soon.
https://russian-dima.wixsite.com/login-system/vnloginmgm
So you can use the shown example and bring it to the next level → using Wix-Secret-Manager.