How do I integrate ctrl and a mouse click?

Hello, I would like to create a freeform slide that mimics an Excel spreadsheet where someone must choose multiple cells. Of course, in Excel that is accomplished by holding ctrl + clicking the cells you want. How can I mimic this in Storyline? There is a basic file attached to play with but it is raw. I've tried a Pick Many Freeform, a Shortcut Key Freeform, a Pick One Freeform with triggers, and more. Where I keep getting hung up is the integration of ctrl. I cannot find a way to simply use ctrl; Storyline only seems to accept ctrl + another key as the shortcut or key press. Does anyone have a workaround?