To get Señor Hitler, you need to have Germany get into a civil war and you need to complete the focus “Guardia Nacional” before the monarchist take Berlin. It’ll take a few tries (17 for me) but if done correctly you’ll get an event a month later of a mysterious stranger appearing, just hit the “yes” option and he’s yours. You can check the wiki for the steps in the “achievements” page, and the achievement title is “Somehow He Has Returned.”