Keyboard access to Button of Tadveditbtn

I have not been able to discover any keyboard means for a user to press the button portion of a tadveditbtn. The only way to press the button seems to be to click it with the mouse. I tried adding an accelerator to the button caption but that does not work. Tabbing from the input field also does not seem to work. Is there a way to do this? If not, could you provide one so that it is possible to use the control in a W7 UXGuide compliant program?

The F4 key triggers this (consistent with F4 action on a combobox dropdown button)