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I didn’t find any reference in the Booking Tools API to create/add custom fields by Microsoft Graph.
Power BI
Open Power BI Desktop. If you don’t have installed this software in your machine, you can open Microsoft Store
Search Power BI Desktop and install it.
Click on Get Data
Select Online Services > Microsoft Exchange Online and click on Connect
Add your booking page address and click OK.
Note: If you don’t know your booking address click on Booking Page and copy the address from the Booking Page Status.
Select Microsoft account > Sign in
Click on Connect
Select Calendar > Load