function myFunction() {
  var body = DocumentApp.getActiveDocument().getBody();
  
  var paragraphs = body.getParagraphs();
  
  for (var i = 0; i < paragraphs.length; i++) {
    var paragraph = paragraphs[i];
    var index = i;
    var text = paragraph.getText();
    if(text === 'key paragraph'){
      Logger.log('index %s text %s', index, text);
      var child = paragraph;
    }
  }
  
  var childIndex = body.getChildIndex(child);
  Logger.log('childIndex %s', childIndex);
  
  //input text to insert
  var ui = DocumentApp.getUi();
  var response = ui.prompt( '挿入する文字列を記入する。', ui.ButtonSet.OK_CANCEL);
  
  // Process the user's response.
  var button = response.getSelectedButton();
  var textToInsert = response.getResponseText();
  //キャンセルならスクリプト終了
  if (button !== ui.Button.OK) {
    return;
  }
  
  //insertParagraph
  body.insertParagraph(childIndex + 1, textToInsert);
}

//メニューを追加
function onOpen(e) {
  DocumentApp.getUi()
      .createMenu('GAS')
      .addItem('insert paragraph', 'myFunction')
      .addToUi();
}