How do I get Untapped.gg to display in OBS while I'm streaming MtG:A?

Open the Untapped.gg Companion settings

Click on the MTG Arena tab and select 'Show player deck window' and 'Enable Streamer Mode'

Add a new Window Capture in OBS, select [Untapped.gg Companion.exe] and select 'Window title must match'

Right-click Window Capture to add Filter, select 'Chroma Key' and color type Magenta in the dropdown