How do I marry my heirs?

Hello friends, following my second campaign now with Scotland, I came across a difficulty that may just be beginner's foolishness, I don't know.

My crown prince is already 32 years old and single, just like his 30 year old brother, I am in turn 20, approximately in turn 10 I had the event to marry him to a suitor but I rejected because I imagined that I could marry him to a French or German princess, who are my two allies.

It turns out that since then I have been visiting the cities and castles of these factions with my diplomats but I never have the option to marry heirs and princesses, am I doing something wrong?

If it is not possible to marry them to princesses, how can I at least marry my heir to a common suitor? Do I need to hope for another event to occur?