@Rachna Harlalka, Thanks for posting in Q&A. For existing Microsoft Entra joined device, there's script to enroll them into Intune automatically. Here is a link list the detailed steps and script for the reference.
https://call4cloud.nl/2020/05/intune-auto-mdm-enrollment-for-devices-already-azure-ad-joined/
Note: Non-Microsoft link, just for the reference.
For Microsoft Entra registered device, you need to do it manually. I don't find the auto method.
Hope the above information can help.
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