How detect Return key in an ActiveX TextBox on an Excel Worksheet?

The ChartObject on my worksheet displays trend lines. The user controls the period of the rolling averages by entering a number between 3 and 21 in an ActiveX TextBox. A TextBox_Change event handler can check whether each character is a digit as it’s entered but VBA can’t check whether the entry is within range and update the chart until the entry is complete. I can’t figure out how VBA can detect the Enter key has been pressed. The Enter key does NOT appear to raise KeyPress or LostFocus events. In fact, I can’t make it ever any raise any events.

There are other ways the user could indicate the entry is complete, of course: Select another control or a worksheet cell which does raise a LostFocus event or I could add a Command Button control next to the TextBox labelled “Update”. For that matter, the user could just enter the value in a worksheet cell and Data Validation could check it. But pressing Enter to indicate an entry is complete is so universal in Excel, I just wonder if I’m not missing something obvious?