Handheld.PlayFullScreenMovie

Test it with a GUI.Button(). With this, you can test if the video launches whenever you want.

You need to check the path or the format of the video. Look at the Documentation.

How to use

 Handheld.PlayFullScreenMovie(m_path, Color.black,
 FullScreenMovieControlMode.Full, FullScreenMovieScalingMode.None);

How to set the path

 if UNITY_ANDROID
     string filename = "yourpath";
     if (!filename.Contains("://"))
     {
         filename = "file://" + filename;
     }
 elif UNITY_IPHONE
     string filename = "yourpath";
     if (!filename.Contains("://"))
     {
         filename = "Data/Raw/" + filename;
     }
 #endif

This is pseudo code and not tested but it should work. Look at the Documentation.