Open the Radeon software -> Settings -> Display, top setting is "Adaptive Sync Compatible", make sure that's enabled.
Then go to the Gaming page and click the game you want to test it in - on the right hand side, under the "Display" section, there should be an "AMD FreeSync" setting, if you can set that to "AMD optimized" then that should do it.
If the monitor has an FPS counter built in, then you can easily check it's working by enabling the FPS counter in both the monitor and in the Radeon software* - if the monitor FPS changes up and down to match the software FPS then it's working
* Performance -> Overlay -> Enable "Show Metrics Overlay"