function setTrigger() {
  var setTime = new Date();
  setTime.setHours(setTime.getHours() + 1);
  setTime.setMinutes(00);
  setTime.setSeconds(00);
  ScriptApp.newTrigger('sendMessage').timeBased().at(setTime).create();
}

function delTrigger() {
  const triggers = ScriptApp.getProjectTriggers();
  for(const trigger of triggers) {
    if(trigger.getHandlerFunction() == 'sendMessage') {
      ScriptApp.deleteTrigger(trigger);
    }
  }
}

function sendMessage() {
  delTrigger();
  setTrigger();
  var options = {
    muteHttpExceptions:true
  };
  var url = 'https://AppName.herokuapp.com/';
  var data = UrlFetchApp.fetch(url, options).getContentText();
}