The same thing works in FireMonkey, as in VCL code - use TComboBox.Items . TComboBox.ItemIndex indicates which one is currently selected (or you can set a selection).
Think:
if ComboBox1.ItemIndex <> -1 then ShowMessage(ComboBox1.Items[ComboBox1.ItemIndex]);
To install:
ComboBox1.ItemIndex := 2;
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