Reference
Customizing with form regions
In a form region, you can set the subject prefix in the XML manifest file of the form region. Alternatively, you can write code in the BeforeFormRegionShow method of the FormRegionStartup interface.
For more information, see How to: Specify a Subject Prefix of an Item Resulting from an Action.
Customizing with form pages
1. In the Forms Designer, click the (Actions) page of your form.
2. Click the action to which you want to add the prefix text, and then click Properties.
3. In the Subject prefix box, type the text as you want it to appear (Outlook automatically adds a colon after the text).
For example, you could add the prefix “Re:” to a reply form.
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