Hi @Annie Tang
Currently there is no ootb method to make digital signature in sharepoint 2013. You may need to use 3rd tool to achieve it which is not supported in Q&A forum.
Here are two similar issues for you:
https://learn.microsoft.com/en-us/answers/questions/369433/looking-for-a-contract-renewal-management-software.html
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/5c2ef9fa-33af-4d81-987e-5bf7a18eafe3/how-to-have-digital-signature-in-a-sharepoint-2013-custom-list-form-created-in-infopath-2013?forum=sharepointcustomization
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