For this tutorial i'm assuming you have installed firebase sdk and log into your firebase account.

Setting up things

1. Open terminal & run firebase init to initilize firebase functions
2. We need two packages for this, add them via npm
   1. npm install @sendgrid/mail
   2. npm install cors
3. Now open index.js and put below code.

const functions = require('firebase-functions');

const admin = require('firebase-admin');
admin.initializeApp();

const SENDGRID_API_KEY = "PUT_YOUR_SENDGRID_API_KEY";

const sgMail = require('@sendgrid/mail');
const cors = require("cors")({
  origin: true
});

exports.sendEmail = functions.https.onRequest((req, res) => {
	return cors(req, res, () => {
  	const msg = {
      to: req.body.email,
      from: 'hi@somedomain.com',
      subject:  'SUBJECT',
      templateId: 'd-02d153b9f48f490bb73bfcc6ad92d892', //SendGrid Template ID
      // SendGrid Template Variables
      dynamic_template_data: {
        name: req.body.name,
        event_name: req.body.event,
        date: req.body.date
      }
    };
    
    sgMail.send(msg).then(res => {
      res.send({
        success: true,
        message: res
      });
    }).catch(err => {
      res.send({
        success: false,
        error: err
      });
    });
  });
});

4. Now deploy our function by running firebase deploy --only functions

NOTE - To work this correctly, you must enable billing on your firebase account