Where is the command that is run when 'Open Link' is clicked?

[Chris Green]

I want to investigate why, when I click on 'Open Link', I get a new
instance of my browser (Vivaldi).

When I was using Firefox and clicked 'Open Link' I'd get a new tab in
my existing browser window.  I want the same to happen with Vivaldi.

When I run Vivaldi with a URL on the command line it *does* open a
new tab in my existing Vivaldi window so it seems strange that this
doesn't happen when clicking 'Open Link'.

So where is the command stored that is run when one clicks 'Open Link'?

-- 
Chris Green