Finally, I was implementing squix78 instructions and script, to get Google Sheets working with the TTN HTTP Integration!
I still have to perform some minor changes to the script, to fit it to my payload, but it works fine.
For educational purposes, it’s good enough 
Many thanks for your tips!
That example is a bit outdated (no setup is needed when creating the Apps Script from Google Sheets) and does not inform you about errors. But indeed it should work just fine otherwise.