This is fairly straightforward e.g.
Option Explicit
Private Sub CommandButton1_Click()
Select Case ComboBox1.ListIndex
Case 0 'First item
AutoTextToBM "BookmarkName", "Template", "Autotext Entry1"
Case 1
AutoTextToBM "BookmarkName", "Template", "Autotext Entry2"
'etc
Case Else
End Select
End Sub
Sub AutoTextToBM(strbmName As String, oTemplate As Template, strAutotext As String)
'Graham Mayor - http://www.gmayor.com - Last updated - 17 Jul 2018
'strBMName is the name of the bookmark to fill
'oTemplate is the template with the autotext - probably ActiveDocument.AttachedTemplate
'strAutotext is the name of the autotext entry
Dim oRng As Range
On Error GoTo lbl_Exit
With ActiveDocument
Set oRng = .Bookmarks(strbmName).Range
Set oRng = oTemplate.AutoTextEntries(strAutotext).Insert _
(Where:=oRng, RichText:=True)
.Bookmarks.Add Name:=strbmName, Range:=oRng
End With
lbl_Exit:
Exit Sub
End Sub