MailMergeFields.Add (Word)

Returns a MailMergeField object that represents a mail merge field added to the data source document.

Add (Range, Name)

ActiveDocument.MailMerge.Fields.Add Range:=Selection.Range, _ 
 Name:="MiddleInitial"

Arguments

The following arguments are required:

Range (Range) - The range where you want the field to appear. This field replaces the range, if the range isn't collapsed.

Name (String) - The name of the field.