Where does Construct 2 look for video files

I am trying to import a video into my game.

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There are options to specify what video to play (Source)

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How do I know where that source directory is, because my video (mp4, and yes, I am choosing the mp4 source option) wont show up when I activate the "Play" action or turn on auto-play

it is a project file so, it looks at the root of the game (he name itself without extension may be enough, due to how relative url works).

this may be of help ( the release notes regarding the release when video was introduced).

also some platform/browsers will not support mp4, when others will IIRC.

https://www.scirra.com/construct2/releases/r171

although I am not a big user of this functionnality.