@Someiah C S Thank you for reaching out to us, If you are looking for instructions how to use dynamic content and work with playbooks, please refer to this - https://learn.microsoft.com/en-us/azure/sentinel/playbook-triggers-actions
If the issue is to find "Assign to me" in the playbook so that it is assigned to the person, would recommend to check is users data returned in response, when you click "Assign to me" and use that field (either dynamic content, or if not available in dynamic content to use expressions). If that data is not available in response, would suggest using text field there to submit the value, and in text field enter UPN, and then pass that to Update Incident action to assign owner.
Let me know if you have any further questions, feel free to post back.