I'm a french student in industrial engineering, working in apprenticeship and I would like to go abroad for my first job. I heard, from another's company employee, they are searching for an industrial engineer for a 2 years mission in USA. However, I can't find this post on their website. I would like to submit a spontaneous application, but how should I write, in my cover letter, I want to apply only if a can go abroad ?
Just do it.
Mention that you heard from another employee about this job. There is nothing wrong with that.
And if the heading of your cover letter contains something like:
Application for industrial engineer for 2 years mission in USA
it is aboundently clear that you only want this job if you can go abroad.