There are two ways you can add Tags to your video: Capture and Edit modes.

 

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Capture Mode

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1) While in Capture mode, visit the Tags section of the Elgato Game Capture HD software.

 

You can use these fields to enter the Title, Game, Description, and Tags for your video.

 

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Edit Mode

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1) While in Edit mode, choose the Group in your Library that contains the video

2) Under the Videos section, highlight your video
 
3) At the bottom, left hand corner of the window, you will see the current title of your video.  Press the small, square button with the icon of a lower case letter "i" on it.

4) A window will pop up, where you can enter or change the Video Title, Game Title, Level, Configuration, Tags and Description.