From Xojo Documentation
The number of highlighted characters.
A SelectionLength 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 SelectionStart 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.SelectionLength = 3