Hello JiHun Oh,
Currently with Microsoft Graph API, doesn't have support for Signature.
Refer to documentation mailboxsettings, which lists mailbox settings that can be managed through Graph.
There is Graph Feed Request submitted, Make email signatures available on Graph you can upvote the request and provide you feedback, and Microsoft Product Engineering team can review and check the feasibility and consider for implementation.
Refer to similar posts: User email signature management via Graph API
Hope this helps.
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