Open the email notifications list in your store's Admin area and proceed to the notification details page. See details described in Editing the Texts of Email Notifications.

Locate the "Dynamic message" field and click the "Edit" button opposite it.

A dynamic message editor will open in a new tab:

In case there is no dynamic content embedded, the page may look as follows:

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In the above notification, the dynamic message is empty. If you need to use a specific TWIG code, use the template tree on the left-hand side of the screen to choose the TWIG file you require. We recommend using the WYSIWYG editor on the "Edit notification" page for plain text.

To edit the dynamic content, click either on a file path to the left or double-click on highlighted content to the right.

This opens a new tab with a twig file code.

Edit the twig code the way you need it.

Click Save changes.