Please hold shift+command and then scale the image or any object so that their proportions are locked.
I hope this is the thing you are looking for.
ah I'm sorry, I mean when I type exactly a number of width or height an other number is scale follow
There's currently no way to do that. Can you please go to UserVoice Adobe XD: Feature Requests: Top (391 ideas) – Adobe XD Feedback : Feature Requests & Bugs and make a request for that feature there?