Mail merge emails do not show up in the Messages tab on entity records

Scenario:
 
The user is unable to see the mail merge emails under the Messages subtab on Entity records.
- The user has sent an email to this entity through bulk merge.
- The email does not show up under the Messages subtab of the entity record.

Solution:
 
The entity form has to be customized to add the Bulk Merge subtab which can be found under Lists subtab> General subtab in the custom entry form page.