email-provider | Detect email provider by address | Email library

You need to generate and send the email via a backend, by using the Admin SDK. The easiest is to use a Cloud Function, from which you use the Mailjet NodeJS API wrapper.

So, in the Cloud Function, you need to:

1. Call the generateEmailVerificationLink() method of the Admin SDK, which returns a link
2. Generate an email containing this link
3. Send the email to the user (see the Mailjet NodeJS API wrapper documentation)

The first steps are detailed here in the doc.

In order to customize the URL of the verification link (e.g. you want to redirect to https://www.myrapp.com/emailVerifyScreen) you need to change the base URL as shown in the below image ("Customize action URL"). Also explained here in the doc.

Then, when the user clicks on the link in the email he/she has received, you need to do what is explained in the doc you referred to in your question: Create custom email action handlers. See the point #4 "Handle email address verification by calling applyActionCode".

Concretely, in the page of your app https://www.myrapp.com/emailVerifyScreen, you get the query string values from the URL (e.g. var actionCode = getParameterByName('oobCode');) and you use these values to call the applyActionCode(actionCode) method. When the promise returned by this method is fullfilled, you know that the email has been verified.