How to send only specified fields? (Not all fields in the page!)

I've created a account page that does accounting like login, register, activate etc. We've multiple forms in this page (login/register..) and we're performing multiple operations. Also, login fields are 'lUsername' and 'lPassword', register fields are 'rUsername', 'rPassword', 'rConfirmPassword' (...).


I want to send only 'lUsername' and 'lPassword' fields for login operation but I see form posting all fields to server (to execute). This is the normal operation for Asp.Net because all (runat="server") asp components/scripts running/executing on server.


I wonder how to send only specified fields in Asp.Net page? I think this is not possible with Asp.Net page itself because it must have only "one" main form (because of ecosystem).


Now, should I use a Javascript? Or what's the solution?


(Namely I don't want to send "all" fields to server with GET/POST requests. I just want to send only specified fields about operation.)


+Additional: I know, we can do it with multiple forms in HTML. Create 3 different forms like <form> Login </form>, <form> Register </form> and <form> Activate </form>.


What's the trick? :)