You can do it with a list box and a rich text (or plain text) content control and a macro.
You may find
https://www.gmayor.com/insert_content_control_addin.htm useful
The macro goes in the ThisDocument module of the document. Change the control titles to reflect what you have inserted.
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim oCC As ContentControl
Select Case ContentControl.Title
Case "DropDownBoxTitle"
Set oCC = ActiveDocument.SelectContentControlsByTitle("Code Text").Item(1)
If ContentControl.ShowingPlaceholderText = False Then
Select Case ContentControl.Range.Text
Case "1": oCC.Range.Text = "Text for item 1"
Case "2": oCC.Range.Text = "Text for item 2"
'etc
End Select
End If
Case Else
End Select
Set oCC = Nothing
End Sub