Hey there @NithinNS 
First things first - a very warm welcome to the monday.com community 
I hope you enjoy your stay.
To be transparent with you, this is not possible within the API tools that we offer currently. I am afraid there is no way to create a subitems column on a board by using the API directly, and instead, this is only possible to do from the UI directly. Does that make sense?
I would really appreciate learning more about your use case here - this would help us understand the impact this could potentially have for you and other teams in a similar position. If you were able to elaborate on this for us, I would really appreciate it.
-Alex