You can make the user-defined field mandatory. If you do this, you need to initially supply a default value, to maintain the integrity of the database. However, if you are working with release 8.82 and later, you can afterwards update the field and deselect the default value checkbox.
Here we have set a default value for a date/time field. The default value will apply if the user does not enter data in the mandatory field.
Be aware that, if you update an existing user-defined field to add a default value to a mandatory field, the default value will be added retroactively to all the documents that exist for the object.