I am still not understanding how fetch requests are structured. In my Chrome Extension, I have used an API key from my console to GET some values from a spreadsheet:

let fetch_url = `https://sheets.googleapis.com/v4/spreadsheets/${spreadsheetId}/values/${range}?key=${API_KEY}`;

It worked, but when I want to append some values, it doesn't:

let fetch_url = `https://sheets.googleapis.com/v4/spreadsheets/${spreadsheetId}/values/Teachers!A7:D8:append?key=${API_KEY}`;
    
let fetch_options = {
                       "method": "POST",
                       "body": JSON.stringify({
                                 "values": [
                                              ["Door", "$15", "2", "3/15/2016"],
                                              ["Engine", "$100", "1", "3/20/2016"],
                                           ]
                                  })
                    };

I get a 401 response, what am I missing?