As you are probably noticing the way you are doing it could lead to issues. What if the user forgets or doesn't know how to bold a selected number? What if they only bold part of it? What if they bold more than one? On top of that it is pretty un-intuitve, I've never seen this as a standard so other users would likely be caught off guard too.
So I suggest three alternatives:
By splitting the inputs and telling users exactly what you want them to put into the boxes it is less likely for them to get confused or mess it up.
As the users enter numbers, split at the comma and dynamically fill a dropdown box. Then users can select which number they want to use as their preferred. You may have to toy with how/when to populate the dropdown to not have it be confusing and to deal with errors.
This method takes up the most room but is the most foolproof. All they have to do is fill in their details and check which is preferred. If they leave it blank then the value is null. You could replace the labels with Contact 1, Contact 2, etc. if that fits your need better. Also this way you don't have to deal with parsing comma seperated values which always finds a way to go wrong.
EDIT: After OP's edit I see I read the problem wrong. I'll leave this here in case it helps anyone else while I think of a new answer.