Welcome to the community, @Linden_H!
The popup that you’re describing is called the “Base Description”, and you can get to it by clicking on the name of your base and choosing “Base Description”.
The “Base Description” will be shown as a pop-up window to anyone who is opening your base for the first time.
When they open the base in the future, the Base Description will no longer pop up, but they can view it again by clicking on the little “i” button next to the name of the base.
Alternatively, as @kuovonne suggested, you can use the Description App which lets you take your descriptions even further — because you can use rich text formatting in the Description App.
Note that the Description App won’t pop up in its own window when you go into your base, but if you leave the “Apps” sidebar open, you can easily see it at all times.