How to request that collaborators add my supervisor's name to a manuscript

I am collaborating with a foreign author. As I am still pursuing my degree, I have to add my supervisor's name in that paper, even though my supervisor has not contributed to the preparation of this manuscript.

How should I ask my collaborator (foreign author) to include my supervisor's name as a third author? I need some help in this.

Edited: As my supervisor has told me to add his name, I am obliged to add. I have no other options left. I am asking how should I write and request to my collaborator to include my supervisor name in the manuscript. Also, I am afraid whether my collaborator will feel odd or bad if I request him to do that.

Please help me: what should I say to my collaborator so that he may give third authorship to my supervisor?