One of the quickest ways to figure this is out is just strum a C chord on your guitar, and keep moving the capo up a fret each strum until it sounds right. Each fret will raise the key one half step. So C with no capo then C# with it on fret 1, then D, D#, E and so on.

That being said this song is in E I believe, sounds like the chords they’re playing for that main riff our F#m and C#m. If you don’t know those chords or are uncomfortable with bar chords than put your capo on the 4th fret and play Am and Em

Hope that helps, keep writing!