User by Company - Asp.net Identity

I would like some help with Identity., please  Here, the user account is by Company, I made the customization of ApplicationUser putting CompanyID field, and the field in AspNetUsers table. Everything working perfect. But now at login time would have to be checked username, password and Company. I have, for example, user "sam" on Company "01" and user "sam" on Company "02". The company I get by parameter in the querystring, and the login screen is still only the UserName and Password fields. It is to do this? Where do I start? Thanks for help.