Hello Georgie Ivanov,
You can set a default value on the entity like this, but when this one is overwritten with empty then it will be empty, so that is not a full solution.
What you can do is to put a event handler and check the field whether it is empty or not and based on that fill it with a specified value.
Another thing that you can consider is to put a validation on the field, so that the user always fill in the value.
Hope this helps,
Good luck!