Should be like this according to AMDGPU - ArchWiki :
Section "OutputClass"
Identifier "AMD"
MatchDriver "amdgpu"
Driver "amdgpu"
Option "TearFree" "true"
Option "VariableRefresh" "true"
Option "AsyncFlipSecondaries" "true" #multimonitor setup, but at least one is not FreeSync capable
EndSection
But, do you have a monitor that is in this list https://www.amd.com/en/products/freesync-monitors
What kernel are you using?