ez asf. put ur vid, set it as bg. then save ur beatmap. reopen it and click file > open .osu/.osb in notepad. then find [Events] and here will be ur vid somewhere at the beginning of [Events]. ya'll find it under //Background and Video events category. there should be smth like Video,0,"vidname.fileformat". then, 0 is ur start time. put there any time in milliseconds (ya can find converters to milliseconds online) and save that file. after that ya should have yer vid at any certain time.
this beatmap should be an example to ya, in this case, since it's utilizing this techinque many times