When clicking 'Add Link' in a Hub Site Navigation bar that has no links you should be able enter a URL and Title for the link in the dialog that appears. You should then be able to save and see the link in the Nav bar.
When adding a link from the dialog the navigation side bar is blank and therefore no link is added to the Hub Nav. To workaround you need to cancel the dialog and then click the + in the side bar to get a new dialog which allows links to be added correctly.
Go to any Hub Site that doesn't have any links in the Hub Nav and click 'Add Link'. Add a URL and title in the box that appears and click OK.
The Hub nav has an 'Add Link' option as there are no existing links. The default dialog shown does not add a link to the sidebar, and by extension Hub nav, after clicking OK.

This works fine as there is an existing link and 'Edit' does not trigger a dialog to add a link automatically.

Thanks for reporting this! I've checked in a fix, which should roll out in the first week of June.
@VesaJuvonen FYI
Awesome news. Thanks!
As of writing 2 x Hub sites created on 13/09/2018 exhibit the same issue - cannot add links to the navigation - the option exists, you get a pop up click save, then links dont update.
Cheers
We are also having this issue. On two different tenants. Cannot add links to navigation. Once we add a url and click save it doesn't update.
Same issue here
Me too.
I was having this issue too. Try adding a link from a document library page or a list page (not a normal page). It opens up the add link in a slightly different spot, but worked for me.
Same issue here too!
Same issue here too. Seems to only happen from the "Home" page. I tried it from the Documents page and it worked! Thanks @pbowens !!
@diver7 I can confirm that @pbowens workaround does in fact work!
Chatted with the team that owns this. There is a bug (as you have pointed out), and a fix has been checked in and should be rolling out across the service over the next week or so. The workaround mentioned here is the best way to deal with the issue at the moment.
same issue here. The workaround mentioned by @diver7 worked.
Yes, workaround worked for me too.
Op ma 17 sep. 2018 om 06:19 schreef maruthigadde notifications@github.com:
same issue here. The workaround mentioned by @diver7
https://github.com/diver7 worked.—
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Thanks for reporting this! I've checked in a fix, which should roll out in the first week of June.
@VesaJuvonen FYI
I See that same issue again in my tenant (dec 2018). Unable to add blank link . @VesaJuvonen @ecraig12345
Trading the workaround (thanks for those that found and posted it), or the intire hub global nav interface for the richness of term store navigation interface borders on byzantine torture! The concept of hub site global navigation is laudable, but the interface is deplorable!
Experiencing this same issue in our tenant. Link to /search.aspx not responsive from homepage, but responsive from other site elements.
Most helpful comment
As of writing 2 x Hub sites created on 13/09/2018 exhibit the same issue - cannot add links to the navigation - the option exists, you get a pop up click save, then links dont update.
Cheers