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I haven't used Open Processing, but you can use an LTI or embed an iframe within Canvas. One of those two things is what you should look for one of those things from Open Processing. I don't see either of those listed on their website (I don't have access to the paid portion), so you may need to just make a link to their site and let it open in a new browser tab. If you want to add a button to the course navigation menu, then you can use the Redirect Tool from the Apps page under your Canvas course Settings.