Paste the code in Game.ini file Path: "Ark/ShooterGame/Saved/Config/WindowsNoEditor"

You may want to Remove the Coelacanth, Leechs, Meganeura, Piranha, Sabertooth Salmon, Titanomyrma or the dino caves in the ini if rexs overspawn in their the map or caves.

You can change the dino id to the id of any other dino if you want just change "Rex_Character_BP_C" in ToClassName to id of any other dino (Entity IDs can be found here https://ark.wiki.gg/wiki/Creature_IDs)