Guys, I've updated the OP and added this this information to the "tips" section:
"In addition to DL and DR it's possible that one of the central defenders also has "Long Flat Throw" PPM and take the throw-ins but in this case some changes must be done to the attacking corner set up: the instruction of the central defender who takes the throw-in must be changed to "Challenge Keeper" and the instruction of Central Midfielder(Defend) must be changed to "Stay Back".
So at one time you can try to train "Long Flat Throw" PPM all your players for DL and DR positions and all your player for Central Defender position and this greatly increase your chances that someone of them picks this PPMs.
For example, Giorgio Chiellini from Juventus is a central defender who has "Long Flat Throw" PPM. There are a lot central defenders who have "14" for Long Throws attributes."