Go into Settings -> About Phone -> Tap 7 times on theBuild Number
Go into Settings -> System -> Developer Options -> Toggle theOEM UnlockingON
Power off the Smartphone
**Boot into Download mode -> Hold "Volume Down" and "Bixby Button" and at the same time Plug-In the Smartphone into the PC ( you can also hold "Volume Down" and "Bixby Button" + "Power Button" till the Screen Flashes ) **
Press "Volume Up" to Unlock the Boot-loader(This will wipe all your DATA)
(Optionally) You will need to Flash a customvbmetato be able to Flash custom Firmware.