How to add more Options under Created Custom Button?

Hello everyone,

I created a new custom doctype called (BOQ) Bill of Quantity. So now, using a custom script, I had created a button named as Create. Now what I am looking for is when a user click on (Create Button) at the top it should list drop down button Options like ( Material Request, Request for Quotation, Purchase Order, Purchase Invoice and Payment entry). Please how can I do this?