To unlock the Autograph Hunter trophy in The Division 2, you will first need to gain access to Specialisations. These become available once you complete the last Stronghold main story mission called Capitol Hill.
Once unlocked, you can find the Specializations vendor on a table next to the Quartermaster at the White House. You can choose between the following options:
Demolitions: Grenade Launcher
Sharpshooter: .50 Cal Sniper Rifle
Survivalist: Crossbow
You can switch to these weapons by holding
.
Choose one of the above, head outside the White House and kill the first enemy you see. Head back to the Specialisations Vendor, switch to a new one and repeat x 2 until you have at least one kill with each one.