Asking a foreign professor to be your thesis co-advisor
Since you clarified that the &quot;official topic&quot; is research and career progression, I would just let it come up naturally. After you have clarified your general interest in the topic and in doing a PhD., there is an ideal spot for something like:

&quot;But of course, before I can do that, I need to finish my master's here by writing a thesis. I think it would be useful if that already was in the same area. Professor X told me that he might be willing to be the official supervisor of this thesis as long as I can find a co-supervisor that matches the specific field better. He suggested asking you, so would you be interested?&quot;

You could do this in the meeting or already in the email setting up the date, though for the latter you should take care that it doesn't get to long, the details are what the meeting is for.