Dropdown as a multi-select

How do i make a dropdown field on a dialog to be a multi-select?
I am working on AX 2012. This particular report is within Accounts Receivable > Reports
I have a DP, a contract and a controller class. I don’t have a UI builder class for this as it is not required for the project. Currently I have a simple dropdown that we can use to select one option.

This blog post may help you: AX 2012: Multi-Select Lookup for SSRS Report Dialog.