This example adds a FILLIN field that prompts you for a name to insert after "Name:".
With Selection
.Collapse Direction:=wdCollapseStart
.InsertAfter "Name: "
.Collapse Direction:=wdCollapseEnd
End With
ActiveDocument.MailMerge.Fields.AddFillin Range:=Selection.Range, _
Prompt:="Your name?", DefaultFillInText:="Joe", AskOnce:=True