Hi,

You need to add your custom field into Messaging_Message table and ensure that the "Messaging.Message" class (CMS_Class table) will be updated to use this custom field.

You can use following code to save the value to your MessageInfo object in code:

MessageInfo mi = new MessageInfo();
mi.SetValue("CustomField", value);

You need also to modify ~\CMSModules\MessageBoards\Controls\MessageBoard.ascx file according your needs, where you need to ensure your functionality.

Best regards,
Miroslav Remias.