ASP.NET Control Access Key, AssociatedControlID and Underline single character for access key

Create a local resource file for the corresponding page.

Make an entry in the resource file Name = lblUserNameResource1.Text
and Value = <U>U</U>sername:

and in the ASP.net page for the control use the meta tag instead of text
i.e. meta:resourcekey="lblUserNameResource1"