Convert Spreadsheet to CSV and send data to API.

send_list_to_api.gs

function sendListToApi() {
  var props, ssId, sheetName, url, params, ss, sheet, range, values, csv;
  
  props = PropertiesService.getScriptProperties();
  ssId = props.getProperty('ssId');  // Spreadsheet ID
  sheetName = props.getProperty('sheetName');  //Sheet Name
  url = props.getProperty('url');  // API URL
  
  ss = SpreadsheetApp.openById(ssId);
  sheet = ss.getSheetByName(sheetName);
  range = sheet.getDataRange();
  values = range.getDisplayValues();
  
  // Convert range to CSV text.
  csv = values.map(function(v, i, arr) {
    return v.join(',');
  }).join('\n');
  
  params = {
    method: 'post',
    payload: csv,
    contentType: 'text/csv'
  }

  UrlFetchApp.fetch(url, params);
}