From Xojo Documentation

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

Supported for all project types and targets.

Gets or sets the text of the current item. Text is read/write for ComboBox.

Sample Code

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

ComboBox1.Text = "Lauraline"
TextField1.Text = ComboBox1.Text