I have an ASP.NET home page where user provides his login ID. Depending on the application (SharePoint Site) the ID is associated to- the user is directed to the respective sharepoint site. 

For example, if user is site1user, the ASP.NET page directs the user to sharepointsite1.

I got it working this far.

Now, how do I pass the login ID from ASP.NET to the SharePoint login page so that the user need not type in user ID again.

I tried URL parameter passing. But it didn't work.