Can I call a custom function in an excel spreadsheet from the microsoft graph?

Question

In the Microsoft graph REST API docs, it shows how to call a function from an excel sheet.

https://graph.microsoft.com/v1.0/me/drive/root:/book1.xlsx:/workbook/functions/pmt
content-type: Application/Json 
authorization: Bearer {access-token} 
workbook-session-id: {session-id}

{
    "rate": 4.5,
    "nper": 12,
    "pv": -1250
}

The example they use is pmt, which is a built in Excel function. It seems odd that the query needs to hit a specific excel file to do that, as the result from that function is totally independent from a specific workbook.

Is this a quirk of excel, that everything needs to hit a specific workbook, or is it possible to make this API hit a custom function?

I've tried to make a custom function in an .xlsm that's in my onedrive:

Function YO(name)
 YO = "yo " + name
End Function

Which works in the spreadsheet, but gives the following error from the API:

{
    "error": {
        "code": "BadRequest",
        "message": "Resource not found for the segment 'yo'.",
        "innerError": {
            "date": "2022-05-13T07:58:00",
            "request-id": "5ca7d80b-e326-4819-abf7-1928d08c599e",
            "client-request-id": "ab896375-ce1f-9ee2-2b08-2fa7d378f007"
        }
    }
}

This leaves me with these questions:

• If this is possible, how would I do it?
• If it's not possible, why does it need a specific workbook to run the function?

The real world use case I can see for this is: If I want to expose a calc that's in a spreadsheet to an external user as an API that doesn't need a developer to build the logic for an endpoint in a fancy way.