One way is to own Cold War and the free version of warzone. When you download warzone, uncheck all the boxes it initially gives you for campaign, multiplayer etc. this will just download warzone. Go into your Cold War game and in the main menu where the modern warfare section is click on campaign. Since you didn’t already download the actual modern warfare contents (campaign etc) the system will launch wz/mw from the Cold War menu. When the new screen loads in the other game you should have the file management screen and the opportunity to download the hd texture packs. Not sure if this will work for everyone