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