ID for page is always __Page

I am registering a callback that I want to access asynchronously. This callback will be executed for validation before form submit. Problem is, since the form action goes to a different page, making a reference to Page results in a reference to "__Page", which gets evaluated as a reference to the target page, and the asynchronous calls go there instead of the current page. (The callback is registered in a usercontrol on the page instead of the page itself.)

I can change the form action on the client-side after I make the calls, but I'd rather take care of it all in the callback registration. So how can I register the callback to something oth开发者_如何学Pythoner than __Page?

tldr: change the action of the form client-side. DoCallback uses the action of the form to make the async request.