Query with checkbox and combobox in Access Database

Good afternoon This is my query:

SELECT Reference.quote_date AS [Quote Date], Reference.agent_ID AS Agent, Reference.ref_ID AS ReferenceID, Reference.ref_number AS [Ref Number], Customer.title_id AS Title, Customer.name AS Name, Customer.surname AS Surname, Customer.postcode AS Postcode, Customer.telephone AS Telephone, Reference.location_ID AS Location, Reference.policy_ID AS Product, Reference.price AS Price, Reference.status_ID AS Status, Reference.source_ID AS Source, Reference.calltype_ID AS [Call Type], Reference.prize_draw AS [Prize Draw], Reference.Call_back_date AS [Call back date], Reference.call_back AS [Call back], Reference.IsCompleted AS Completed, Reference.comments AS Comments FROM Customer INNER JOIN Reference ON Customer.[customer_ID] = Reference.[customer_ID] WHERE (((Reference.agent_ID)=[Forms]![Call_Back_Search1]![Combo5]) AND ((Reference.Call_back_date) Between [Forms]![Call_Back_Search1]![StartDateTxt] And [Forms]![Call_Back_Search1]![EndDatetxt]));

It Currently shows all call backs within certain criteria selected in Call_Back_Search1 form: - date criteria: between Start and date:End - and allows to select the Agent (Combobox05). I wish to add additional criteria to my query :
checkbox called "Completed" to be not ticked - to show results based only on date range selected (leaving combobox blank)
Wonder if I can ask you for any suggestions Thank you