The usual technique is for the performer to wear headphones.

If you are saying that synchronisation with the original material will be carried out after the recording, then an easy thing to do is to transfer the track in advance to an MP3 player which the singer listens to on headphones while the computer records her singing.

If you want the synchronisation to be automatic, transfer the backing track to the computer first and get Cubase to play it via the Audiogram headphone output while recording the microphone channel.

There are other techniques that do not involve headphones. One of these is to use a loudspeaker and amplifier in the studio area to play the backing material from Cubase via the Audiogram interface. You first record a track with the backing material playing and the singer standing in place but not singing. You then record as many singing takes as needed with the background track playing. Make sure everything is in exactly the same physical place for these takes and the volume of the monitor loudspeaker is not changed. In Cubase, you subtract the first silent vocal track from each of these, and you are left with vocal tracks with no audible background material on them. You choose one of these tracks to mix with the original backing track as your final result.

Is the interface an Audiogram 3 or 6?

Its the audiogram 3;)
But the best way is to ear headphones? Isnt it strange to wear headphones on a cover video?

That's why I suggested that there are other techniques. On the other hand, I have yet to see a promotional video that is not showing the performers miming to a track previously recorded in a studio.

So in your opinion is better to use headphones (or in-ear)?
Btw the phones have to be connected to the audiogram or to the pc/mp3?

Take the easy route and do a studio audio recording using good closed-back headphones connected to the Audiogram. After you have got the best audio track you can, do a separate video-only take with the performer singing or miming to the track being played via a loudspeaker but with no microphone present so no audio being recorded. Then line up the video with the audio and you are done.

Thanks mate,, but one more question, will people notice that that video dont match the audio, or its very hard to see that?

Whether people will notice or not is up to you - it's your job to make it match!