Google Sheets script to add rows via POST request (JSON or form payload). Only accepts fields that are valid columns on the sheet.

post_sheets.gs

/* === Configuration === */

// The sheet to work with
const SHEET = SpreadsheetApp
            .openById("FILE ID")
            .getSheetByName("SHEET NAME");

// Create list of headings
const COLUMNS = SHEET.getRange(1, 1, 1, SHEET.getLastColumn()).getValues()[0].map(v => v.toString().toLowerCase());

// The email to send to (if enabled)
const EMAIL = {
  enable: false,
  to: "[email protected]",
  subject: "New Entry!",
};

/* === Functions === */
const doGet = () => {
  return ContentService.createTextOutput("API is working").setMimeType(ContentService.MimeType.JSON);
}

const doPost = (request = {}) => {
  const { postData: { contents, type } = {} } = request;
  let entry = Array(COLUMNS.length);

  // Handle JSON or form inputs
  switch (type) {
    case "application/json":
      Object.entries(JSON.parse(contents)).forEach(([k, v]) => {
        insertIfColumn(k.toLowerCase(), v, entry, COLUMNS);
      })
      break;

    case "application/x-www-form-urlencoded":
      contents
        .split("&")
        .map((i) => i.split("="))
        .forEach(([k, v]) => {
          insertIfColumn(k.toLowerCase(), v, entry, COLUMNS);
        });
      break;
  }

  // Make sure there was valid data to enter
  if (typeof entry !== "undefined") {
    SHEET.appendRow(entry);

    if (EMAIL.enable) {
      MailApp.sendEmail({
        to: EMAIL.to,
        subject: EMAIL.subject,
        htmlBody: "Entry content: " + JSON.stringify(entry),
      });
    }
  }
  
  // Return the data entered
  return ContentService.createTextOutput(JSON.stringify(entry)).setMimeType(ContentService.MimeType.JSON);
};

// Helper function to to build array based on columns
const insertIfColumn = (key, value, array, columns) => {
  if (columns.includes(key)) {
    array.splice(columns.indexOf(key), 1, value);
  }
}