Record-Security Rules/Expressions

Hi! I was trying to make a security rule for my record (filter --> loggedUserName() = nomProveedor (one of my fields)), but in the security rule I can only put direct values or constants

 

I found out that there are security expressions, but how should be my structure to obtain a filter like the mentioned one? Example:

Username: Proveedor 1

I want to filter the record so that he can only see the rows in which my field "nomProveedor" is equal to the Username

Thanks a lot!

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