Hello @SandeepN96,
Thank you for reaching out to Constant Contact's API Support.
You did not mention what error you are getting, but the most likely reason you are having problems is you are using the wrong endpoint. In order to move an existing contact in to a different list you will need to update the contact using a PUT.
https://developer.constantcontact.com/docs/contacts-api/contacts-resource.html?method=PUT
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