how to set watermark to combobox

i want to set water mark to combobox. but i dont know how it is possible? i tried text="select city" but i bind payrollmodel to text of combo in xaml. like this: text="{binding payrollmodelobj.city}" and by another way i tried through the backend in c# like- citycombo.text=("Select City"); but it is shown when i remove text property of combo from xaml.but i want text property in xaml for binding...

 

if any one have any idea how to set watermark to combobox