Greetings from Syncfusion.
We have checked the requirement in SfComboBox control, and you can achieve this by using SelectedItem API. We have prepared a sample based on your requirement. Please have the sample from the below link,
Watermark="Select an Item"
void Handle_SelectionChanged(object sender, Syncfusion.XForms.ComboBox.SelectionChangedEventArgs e)
ObservableCollection<object> SelectedItems = new ObservableCollection<object>();
SelectedItems = (sender as SfComboBox).SelectedItem as ObservableCollection<object>;
Please check with the sample and let us know if you have any concern.
Muneesh Kumar G.