Retrieve the loginuserid using the corresponding loginuser ?

Retrieve the loginuserid using the corresponding loginuser ?

Hi,

I use a form to dynamically share forms/views with users. I just have to fill the "Email" and use it for the sharing details. I'd like to add a field like "zoho_username" to be autofilled according to the the email (registered in Zoho) :
I fill "myUsername@myEmail.com" then my field "zoho username" should be filled with Deluge.

Is there a way to do it please ? (I precise that all accounts will always be created first in Zoho). Something like :
myUsername = zoho.username(input.Email) ;

Actually I use the left part of the email, before @ but sometimes the info is not enough (when we just have info@myEmail.com, admin@ , contact@ etc...)

The revert could also be nice, retrieve the email according to a known Zoho username....

Thanks

Cyril