This is a firebase cloud function example that sends a custom action email through Sendgrid.

sendCustomActionEmail.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const sgMail = require('@sendgrid/mail');

// Using @sendgrid/mail
// TODO: Configure the `sendgrid.key` and `sendgrid.template` Google Cloud environment variables.
const API_KEY = functions.config().sendgrid.key;
const TEMPLATE_ID = functions.config().sendgrid.template;
sgMail.setApiKey(API_KEY);

// Sends an email confirmation when a user is created.
exports.sendVerificationEmail = functions
  .auth
  .user()
  .onCreate((user) => {
    admin.auth().generateEmailVerificationLink(user.email)
      .then(async (link) => {
        // Construct email verification template, embed the link and send
        // using Sendgrid.
        const msg = {
	    to: user.email,
            from: sender.email,
            templateId: TEMPLATE_ID,
            dynamic_template_data: {
                confirmationLink: link,
                username: user.displayName,
            },
        };
	return sgMail.send(msg);
      })
      .catch((error) => {
        console.error(error);
        return '200';
      });
  });