Please find attached a small sample that shows what I would suggest in such scenario. Basically, the idea is to use the OnClientShow event handler of the window and in it, to find the UserName control in the asp:login one. Once you get a reference to its client-side object, you can call its focus() method.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.