To use a 3rd party flash on a Fuji X-T20, go to the menu for "Flash Setting." Choose "Flash Function Setting" and press the right selector button on the back of the camera. This brings up a screen called "Sync Terminal." In the upper left is a box that says either "Off" or "M." If it's "Off," turn the Rear Command Dial to bring up "M." This will also open a small panel called "Sync" in the lower right of the screen. Navigate to it via the selector buttons, then use the Rear Command Dial to choose front or rear curtain sync. Press the "Back" button to exit.