From Xojo Documentation
The position of the insertion point. A value of zero means that the insertion point is before the first character.
Use SelectionStart to set the position of the insertion point in the text. Use SelectionStart in conjunction with SelectionLength to select a portion of the text in the ComboBox, beginning with SelectionStart and extending for SelectionLength characters.
This code selects the second through the fourth characters. After the code is executed, those characters are highlighted in the ComboBox.
ComboBox1.SelectionLength = 3