From Xojo Documentation
| This item was deprecated in version 2019r2.
Please use ComboBox.SelectionLength as a replacement.
The number of highlighted characters.
A SelLength of 0 means an insertion point rather than a selection. A value greater than the number of characters in the ComboBox means that the selection is from SelStart to the end of the ComboBox.
This code selects the second through the fourth characters. After the code is executed, those characters are highlighted in the ComboBox.
ComboBox1.SelLength = 3