Add a Combobox to xaml with more than 1 CombBoxItem and bind SelectedIndex to a ViewModel property
Create Get and Set UFUNCTIONs in C++
Add breakpoint or logging to Set function
Run project and change Combobox selected item
Notice Set not being called when selecting first item

WPF
<ComboBox SelectedIndex="{Binding TestSelectedIndex, Mode=TwoWay}" >
<ComboBox.Items>
<ComboBoxItem Content="1"></ComboBoxItem>
<ComboBoxItem Content="2"></ComboBoxItem>
<ComboBoxItem Content="3"></ComboBoxItem>
</ComboBox.Items>
</ComboBox>

C++
UFUNCTION()
void GetTestSelectedIndex(UPARAM(ref) int32 OutTestSelectedIndex) const;

UFUNCTION()
void SetTestSelectedIndex(int32 Index);