You could prevent the request initially by using the event argument preventDefault method and then when OK is clicked and the login succeeds, manually trigger the request with the dataSource read method. You will also need to use a flag or dynamically bind and unbind the requestStart handler so that the request is not prevented when calling read. I created an example
that demonstrates the basic logic.
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 Feedback Portal
and vote to affect the priority of the items