comboBox- data bound item

Question

Hi :) I have a comboBox and I used the option Use Data Bound Items. I've set the Data Source, the Display Member and Value Member, but I need the comboBox to contain only the clients whose name begins with 'RO'. I've tried to use a Query but it's not correct and I don't know how to do it. Here's the query used:

SELECT denumire FROM dbo.client WHERE denumire.Substring(1,2)='RO'. I know that i should use parameters but i don't know how. Some help?