Public Sub AutoFind()
    On Error GoTo myErrorHandler

    Selection.HomeKey Unit:=wdStory

    Dim theDialog As dialog
    Set theDialog = Application.Dialogs(wdDialogEditReplace)
    theDialog.Find = "the"
    theDialog.replace = "an"

    theDialog.Show
End Sub

Sub TestMacro()
Selection.Find.ClearFormatting
    Selection.Find.Replacement.ClearFormatting
    With Selection.Find
        .Text = "test find field"
        .Replacement.Text = "test replace field"
        .Forward = True
        .Wrap = wdFindContinue
        .Format = False
        .MatchCase = False
        .MatchWholeWord = False
        .MatchWildcards = False
        .MatchSoundsLike = False
        .MatchAllWordForms = False
    End With

    CommandBars.FindControl(ID:=313).Execute
end sub

SendKeys "^h"   ' Hack to obtain "Dialogs(wdDialogEditReplace).Show" modelessly.
' MS Disclaimer: SendKeys is unreliable as to there being no certainty as to where focus is when called.