This example inserts the string " Text to trim " at the beginning of the text in shape two on slide one in the active presentation and then displays message boxes showing the string before and after it's trimmed.
With Application.ActivePresentation.Slides(1).Shapes(2) _
.TextFrame.TextRange
With .InsertBefore(" Text to trim ")
MsgBox "Untrimmed: " & """" & .Text & """"
MsgBox "Trimmed: " & """" & .TrimText.Text & """"
End With
End With