Search a table colomn for a value found using a function

I have a database that currently uses a function to find the windows userlogin that they use.

I have that working fine however I am trying to set up an if function for when a button is clickedit to search for the userlogin within a table, and if it exist than it opens a different form than if not.

I know that the rest of my code works as I have used:

If fOSUserID = "lyona1" Then
...

it works, but want for it to search the column instead of having to add every user individually as these users may often change.