As per the documentation only these properties are available with Microsoft Graph API. AssignedTo
property is not yet available using Graph API. I found one similar feature request idea please upvote it using this support link, which will be monitored by Microsoft team and make the enhancements to Microsoft Graph APIs. I will also upvote for you.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote. If you have any further questions about this answer, please click Comment.