How to add event handler to AJAX Control Toolkit Extenders

Question

I have been trying to add a toggle button using the ToggleButtonExtender and the MutuallyExclusiveCheckboxExtender. However, I cannot add an additional client side event handler for the click event of the Extender created button. I tried to add a BehaviorId that I access in the onLoad of the Javascript via the $find('') function. However, as I search the content of the function, I did not see any method that seems to add an event handler to the function. I did see _checkChangedHandler() but there is no documentation about it.