From Xojo Documentation

Property (As String )
aComboBox.Value = newStringValue
StringValue = aComboBox.Value

New in 2019r2

Supported for all project types and targets.

Gets or sets the value of the current item. Value is read/write for ComboBox.

Sample Code

This code sets the value of the currently selected item and then displays it in a TextField.

ComboBox1.Value = "Lauraline"
TextField1.Value = ComboBox1.Value