Choose a default Space so Giraffe knows where to send you. Once you've picked one, use it as a quick way to jump back to that Space from anywhere.
Star a Space on Home
Open Giraffe. You land on Home.
Select Spaces from the dropdown, in table view.
Hover over the Space you want, and click the star that appears in its row.
The star only appears in table view, on hover. Gallery view just shows a filled star on your current default; it isn't clickable there.
Set it from your profile menu
On Home, open your profile menu at the bottom of the left sidebar.
Open Default Space.
Choose a Space from the list.
Either method sets the same thing: it's what drives the Space icon shown in the breadcrumb trail in the top bar while you're inside a project. See Open a Space.
The profile menu method lives in the same menu as your default workspace. See Account Favorites.