function showNewsPrompt() {
  //Initializes global variables
  ss = SpreadsheetApp.getActiveSpreadsheet();
  ds = ss.getSheetByName('Sheet1');
  headerRow = ds.getDataRange().getValues()[0];
  sentimentCol = headerRow.indexOf('Sentiment');
  headlineCol = headerRow.indexOf('Headlines');
  scoreCol = headerRow.indexOf('Score');

  // Builds Menu
  let ui = SpreadsheetApp.getUi();
  let result = ui.prompt(
    'Enter news topic:',
    ui.ButtonSet.OK_CANCEL);

  // Processes the user's response.
  let button = result.getSelectedButton();
  topic = result.getResponseText();
  if (button == ui.Button.OK) {
    analyzeNewsHeadlines();
  } else if (button == ui.Button.CANCEL) {
    // Shows alert if user clicked "Cancel."
    ui.alert('News topic not selected!');
  }
}
Prompts user to enter a new headline topic. 
Calls main function AnalyzeHeadlines with entered topic.