Hello, I am setting up a growthbook implementation for a client of ours that has a Big Commerce Web store (similar to Shopify) I have followed the set up instructions, but it's a little unclear to me how to actually create a functional experiment where--say-- I want to test --> "Display a progress bar in the side cart to indicate purchase value required to attain discount shipping". I know how to build this in my theme files, but how do I tie it to an experiment?