Is it possible to use the custom fields as a placeholder inside email to fill some data dynamically. So that the creator of envelope just fills in the information in the custom field and it will be used in the email body/subject etc..

Hello, @JohnDeo 

Welcome to the Docusign Community!

Using Envelope Custom fields will permit to you use the data in Search and Report operations, but you cannot merge them in the Subject or Body message.

An option to explore:

1. Using RESP API (you need a developer to do it)
   https://developers.docusign.com/docs/esign-rest-api/reference/envelopes/envelopes/create/

I hope that helps.

Alexandre