You may refer to the link below.
Hope this helps.
Thank You !
If i understand correctly, you want to use the child elements of an entity in mail merge.
But in Mail Merge you can only use the fields of a parent record.
So if you want to use the fields of the custom entity, in your case, register the mail merge against this entity, and then use the related fields from the opportunity.
Hope it helps.
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Thanks, your are referring to use mail merge from reverse direction. However this issue is that there is no mail merge button on the custom entity. Have you done such thing before, if so, how did you deal with the mail merge from custom entity when there is no button available.
Many thanks in advance
This would be a separate issue then.
In CRM 2013 to increase better visibility and UI, some buttons are by default hidden, so you will need to show them manually.
Refer to the following link for restoring a hidden button in CRM 2013.
However, the button may be hidden from the custom entity form, but you should be able to run mail merge, from the list view of the custom entity records, or in advanced find.