Default button for 'Enter' key with a form which does not 'runat="server"'

I just created an ASP.NET, C# web application and for the first time i used a master page.

I learnt that you can't have two forms that runat "server" because it causes an error, so for the form (which uses for login) at the top of the master page i used a runat="server" and for a form in the content page i removed the runat attribute.

Now i cannot use the 'defaultbutton' attribute and the "Enter" key will go to the login button at the master page even if a text field in the content page is now active.

What is the solution to this problem? Thanks!