Payment integrations: How to add property with multiple options to a product?

I am trying to add products with multiple custom options. When doing so, in the actual form, it lists each option as its own drop down rather than allow a selection of several options from One drop down. For example, I am giving cake flavor options and each flavor is listed as a separate drop down rather than all in one so that the customer can choose their cake flavor. 

When you are on the products section and adding the option, just select Add a Custom Property.

Then you will be able to add the property and the options available for it.

Hope this helps.