Confirm Box Code on server Side

I want to prompt one confirm message  based on some condition.

But my question is when I add button.Attributes.Add() in page load it will add the confirmation to button 's click event

Now in button's click event itself I am checking one condition and if that condition is true only then i want to prompt for confirmation and if it is "Yes" only then I want to execute some code .

If I remove onclick event from button  in the button code itself it will not work

Can you please help me, how to solve this issue