how we can add a clickable button on the top navigation bar inside our SharePoint modern pages

[john john]

I have a modern communication site inside our SharePoint Online tenant, and we want to do the following modification to our top navigation area:-

1. Add a button on the top-right corner (the one with green color on the below image) >> and when users click on this an external link will open.

Here is what we need to build:-

So can anyone advice how we can achieve the above 2 points?

Thanks