Lap Define "HOLD" Time?

How long does one have to HOLD the button to set Start/Finish when the button is set to “HOLD”?

I want to add a message screen when the Start/Finish set is activated. But want to know what the timing is?

A button hold is recognized after 500 ms. However, you can simplify and make this process more robust by utilizing the adu.track.userButtonDefinedTrackAcquiring channel. The detailed method is explained in section 19.5 of the ADU Manual: https://www.ecumaster.com/files/ADU/adu_manual_en.pdf. You can adjust this approach to achieve your desired result.