Okay been a while but set the reaction time from the HQ screen
For those you want to remain in place set it high for those you want to attack set it to zero or the turn you want them to move. Note if I remember correctly scouts tend to ignore it so set scouts including scout vehicles as reinforcements entering when you want them to start moving.
Dont think it works for helos either though you can slow them down to some extent using waypoints. Read the help forget think move through a max of 3 waypoints a turn & they will go off & do there own thing once they engage.
If you have not already bought the forces says you shouldnt but if you buy & deploy all holding forces with battle set as delay. Make all adjustments range reaction time etc & only then switch the battle to engagement. Now deploy the other units they will have a reaction time of zero saving you adjusting all of them individually. You can now set them as reinforcements or deploy them & set the reaction time for when you want them to adavnce. Note this way they will NOT got entrenchments
in a defend battle.
Do set some arty as reinforcements timed to arrive with your attacking force.
Important save often if using waypoints & its best to finnish all purhases & deployment before plotting waypoints if you use them, occasionally they can get mucked up if you dont do this.
Suggest try a test deploy switching from delay to meeting with a few units to see how it works first
Hope this helps perhaps someone who has dabled more recently can chip in.