I tried modifying WorkPackage using the API api/v3/work_packages/{id}/activities uses postman. I have used the POST method by sending a JSON containing the payload I want to modify, and it works.

And how to use that API from MS.excel VBA/Macro?

Hello Benny,

have a look at this: https://github.com/opf/OpenProjectExcel

It could help you getting started using the OP API from Excel