The normal way to use commands in Opus is to create buttons (or menu items, hotkeys, context menu items, etc.) which run them. How to do that is well documented and easy to find both (in long form) in the manual and (in summary form) in the FAQs here at the forum.
In this case, the OP wanted to avoid creating a button and to run the command ad-hoc.
As it turns out, there is indeed a way to do that, and the forum provided the answer very quickly. But it's an advanced, esoteric feature which isn't essential to using Opus and it wasn't needed to get the job done; it just provided an alternative way to do it. (If the command field didn't exist or if you didn't know about it, it's still very quick & easy to create a button, use it, then delete the button. Even knowing about the command field, that's usually what I do myself, because the button editor makes building commands a lot easier than typing them from memory into the command field.)
Esoteric features aren't always going to be easy to find, and won't always have How-To tutorial style explanations of how to use them (like there is for mainstream features like button editing). Making esoteric things easier to find often means making the more important mainstream things harder to find, because people will only look through so much in the manual, FAQs and search results before they give up. It's a call we have to make. People often assume everyone else will look for the same things they were looking for, but we get to see all the questions people ask, which gives us a good overview of what people are having problems finding as a group.