*Prerequisite is the "Recruitment Blues" quest to unlock the 4th Floor at Fort Defiance.
Look for a note "Mission 099-01 Orders" next to a terminal to start the quest. The quest sends you off to Vault-Tec University, where you have to go to a Research Terminal. You have to restore power to it by hacking a terminal in the basement. Once you do, the quest tasks you with collecting a DNA Sample from a Scorchbeast.
Fast Travel to a location where spawns are frequent (Cranberry Bog, near a Fissure Site, etc.), kill one and then hit Y over its corpse to collect the sample. Alternatively if you find a corpse in the wild, you can skip the fighting part (the front yard of Whitesprings as the guards tend to shoot one down every now and again).
Return to the Research Terminal at Vault-Tec University to finish the quest.
*This is a Drop Quest, which means it has to be done in one sitting, so if the game crashes, the server kicks you or you leave, you must start over from the beginning.