You have the source code to tk2dButton - you can easily move the lines which trigger the button press
if (targetObject)
{
targetObject.SendMessage(messageName);
}
... to wherever you need it to be.
Eg. If you want to trigger as soon as its pressed, move it to the top of that function, right under the first occurrence of this:
if (ButtonDownEvent != null)
ButtonDownEvent(this);
Or you could use the 2D Toolkit UI button class, which has event triggering for the other options too.