Assuming that you are putting this above the door. Attach like so.
1. Pre drill the 3/4 ply w/ 1/4" bit-3/4" from the top.(4 or 5 evenly space)
2. Put the ply where you want it above the door and then drill thru these holes, making a mark.
3. Pre drill the the plate making pilot hole for 1/2" lags. 4.Attach hangboard to ply.
* Just make sure to drill pilot holes or you'll split the wood and then yous be fucked.