Can we retrieve the Definition ID for Custom fields created after 3 already created in API Explorer for integration purposes?

It's a pleasure to have you here, Krati.

To help retrieve the definition ID for custom fields created in API, let me route you to the best support available. We have a forum called Intuit Developer where you can get an answer from one of the experts handling integration like API, developments, and other related concerns.

I'd suggest posting your situation on their forum, as they are the best gurus who can help you with your concerns. I'd be glad to guide you on how:

1. Proceed to this link: https://help.developer.intuit.com/s/
2. Choose QuickBooks Online.
3. Select Ask a Question.

You'll want to check these articles to help set up custom fields for your sales forms and purchase orders in QuickBooks Online: Add custom fields to sales forms and purchase orders.

Please know that you're always welcome to post if you have any other concerns with retrieving definition IDs for custom fields. I wish you and your business continued success.

Unfortunately, retrieving the Definition ID for custom fields created after the first 3 directly through the API Explorer is not possible. The API Explorer only provides a limited view of data and functionality. To access Definition IDs for later custom fields, consider exploring other methods like querying specific entities that utilize these custom fields or leveraging the Preferences endpoint if available.