Pass JSON to index

Problem/Motivation

I have search api and external search server. To pass more structured code, I want to pass JSON to this server, but there is no option to use JSON n, instead this, it takes the string, and key/values on server are string.

Steps to reproduce

Add custom field, for example image, that has source, alt and title in JSON format and add this field to index.

How should I pass the JSON data to this external server? Is it custom datasource, or item type?
Please if possible show example.
Thanks