External Custom Field Values filtering

Hi, I would need to load a custom field value from an external database but depending on a initial string, because the list of possible values is about 100K (persons).

I already have a external CustomFieldValues working on other field so I know the basics.

I need something like the e-mail input fields, you write 4 letters or so and the system suggest the email that match that criteria. I would introduce the person ID and the system would use it to search the database and get a string with person data.

Other possibility is that I write the person ID on one field and another field “reacts” to some event and use the first field to search the database and get the information.

Any idea?

Best regards,
Ernesto Silva.