Note: this should work for any input that sway recognizes as a tablet input device that provides absolute position instead of relative.
In desktop environments like KDE, there is a fun GUI to help run this. However, in sway, you need to do this manually.
You can view a list of inputs to make sure your tablet is detected:
swaymsg -t get_inputs
Map the output using swaymsg:
swaymsg input type:tablet_tool map_to_output DP-2
where DP-2 is the display to map to. Input type tablet_tool should match all tablets.
I have a video of this I should upload…