You can't actually add buttons to the top banner, but you can make the whole top banner a link to something -- if you go to edit your series (go to your dashboard, click the little pencil icon on your series) then right under where you add the Series Cover Banner is a space for the "Series Cover Banner Link," which you can leave blank or set to anything you want. A lot of times you can click someone's top banner to go to their patreon or their main website.
btw - @ratique, if you feature RW this week it'll be a little tough to compare since I just got a flurry of activity from being in the Holiday Collab since my last update (i.e. "did views go up this week because I have more subs now or because Ratique linked me") but I'll do my best to share how the banner feature affects me whenever you do! Thank you so much again for the rec!!
As for me, I'm not sure how I feel about the idea of adopting this! There's such a big difference between saying "I really recommend this good comic as a creator!" and associating that comic's brand with your own. I guess it's something to ponder!