ASP.NET Default button

I have a master page with a search box and button at the top. This search functionality is taking over the "enter" key for all my web forms that use this master page. That is, if I have a login page that uses this master page and the user enters in their username/password and hits "enter", instead of logging in the user the system performs a search.
When I enter something in the textbox and hit enter, it should search but when I enter password and hit enter it should log me in. How can I handle those different button and make default one in a same ASP.NET page ?
 
Any help is highly appreciated!
 
Thanks
 
 
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