How to add an MX Record in cPanel using DNS Zone Editor:
1. Login to your cPanel, at the Domain section Click on Zone Editor

2. Click on MX Record in the records listed

3. A pop-up opens. Enter the following details i.e. Priority and Destination  (e.g. Priority : 0, Destination: mail.shostdemo.tk)
and click on Add an MX Record.

4. MX Record is successfully created.
For verification, Click on Manage beside the DNSSEC:

5.Scroll down to see the Record you added

Done:.