Hi Eleni
My name is Andre Da Costa; an Independent Consultant, Windows Insider MVP and Windows & Devices for IT MVP. I'm here to help you with your problem.
When you click on New Messsage, then click in the 'To' field, you should be able to type in the contacts name separated by a semi colon then followed by the other contacts name.
Here is a tutorial:
https://www.tenforums.com/tutorials/119892-send...
Information in the above link is sourced from a trusted Microsoft MVP blog.