From Xojo Documentation

Property (As Integer )
aComboBox.SelectionLength = newIntegerValue
IntegerValue = aComboBox.SelectionLength

New in 2019r2

Supported for all project types and targets.

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.

Sample Code

This code selects the second through the fourth characters. After the code is executed, those characters are highlighted in the ComboBox.

ComboBox1.SelectionStart = 1
ComboBox1.SelectionLength = 3