I’m afraid this is not possible in MS Word, so also not in ClauseBase.
As you can see in the attached screenshot from MS Word, you can only choose between a space, tab or nothing after the article number. In the screenshot you sent, you can see that there is a paragraph mark (the reverse P in blue) between the number 1 and the word “Formation”. So if my estimate is correct, the entire numbering is hardcoded.
While I would personally stay away from this styling (besides the inability to use automatic numbering, you also have to take quite some precautions in Word to avoid that titles would end up twice in a TOC), there is kind of a solution possible in ClauseBase.
You can remove all automatic numbering, and then hard-code the word “Article” and use the
@marginal special function that simply inserts an incremental number every time. Be aware, however, that it also inserts a period after the number (the function’s typical use case is to be put in front of sequentially numbered paragraph, e.g. in a legal brief) and that you cannot use cross-reference towards it.