function onClickFunction_(e) {

  const sourceScriptId = e.formInput.sourceScriptId;
  const targetSpreadsheetUrl = e.formInput.targetSpreadsheetUrl;

  // Validates inputs for errors.
  const errors = [];

  // Pushes an error message if the Script ID parameter is missing.
  if (!sourceScriptId) {
    errors.push('Missing script ID');
  } else {

    // Gets the Apps Script project if the Script ID parameter is valid.
    const sourceProject = APPS_SCRIPT_API.get(sourceScriptId);
    if (!sourceProject) {
      // Pushes an error message if the Script ID parameter isn't valid.
      errors.push('Invalid script ID');
    }
  }

  // Pushes an error message if the spreadsheet URL is missing.
  if (!targetSpreadsheetUrl) {
    errors.push('Missing Spreadsheet URL');
  } else
    try {
      // Tests for valid spreadsheet URL to get the spreadsheet ID.
      const ssId = SpreadsheetApp.openByUrl(targetSpreadsheetUrl).getId();
    } catch (err) {
      // Pushes an error message if the spreadsheet URL parameter isn't valid.
      errors.push('Invalid spreadsheet URL');
    }

  if (errors && errors.length) {
    // Redisplays form if inputs are missing or invalid.
    return createSelectionCard(e, sourceScriptId, targetSpreadsheetUrl, errors);

  } else {
    // Calls shareMacro function to copy the project.
    shareMacro_(sourceScriptId, targetSpreadsheetUrl);

    // Creates a success card to display to users.
    return buildSuccessCard(e, targetSpreadsheetUrl);
  }
}
Action handler that validates user inputs and calls shareMacro_
function to copy Apps Script project to target spreadsheet.

@param {Object} e - Add-on event object.

@return {CardService.Card} Responds with either a success or error card.