It looks like they are potentially hooking into <a> link elements instead of listening to window.location change events which is why using go to URL with the #etage anchor isn’t working. There’s probably two options:
Use the inner HTML of the element in Hype and create a <a href = "#etage">web</a> tag. This may not work depending on their hook construction.
It would not be trigger custom behavior. You’d just take an element, and choose Edit > Edit Element’s Inner HTML and then add that text and see if it works. (But like I said, there’s a good chance this won’t).
It may be that you have an error here, the spacing is kind of weird. I’d try the exact line that I posted above, especially since the anchor tag seems to be #etage. That said I don’t have a setup to try this, so it is only speculation! There are definitely other libraries folks have used for smooth scrolling; here’s the previously link but I’d guess there are other solutions.