Specify 'Custom Data' fields for media field

I would love to be able to specify the which field names are allowed for custom data fields.

For example, I have a media field for a video. In my code, I can set the thumbnail for that video based on a timestamp. So I need a field for my editors to enter that timestamp, which the ‘Custom Data’ fields can achieve.

However, the users must enter the custom data field name exactly as I am trying to query it. Without being able to set the default name, I have to rely on the user knowing that they should enter thumbnailTime as a custom data field.

I propose to add a validation field, similar to the “Accept only specified values” validation for text fields: