From Xojo Documentation
| This item was deprecated in version 2019r2.
Please use ComboBox.SelectionStart as a replacement.
The position of the insertion point. A value of zero means that the insertion point is before the first character.
Use SelStart to set the position of the insertion point in the text. Use SelStart in conjunction with SelLength to select a portion of the text in the ComboBox, beginning with SelStart and extending for SelLength characters.
This code selects the second through the fourth characters. After the code is executed, those characters are highlighted in the ComboBox.
ComboBox1.SelLength = 3