To make it sticky is the same procedure to move an app to a different desktop.
Select the desktop to move there, then popup menu and select the app.
Sticky desktop only shows when more than one desktop has been selected.
flwm always starts with minimal resources, i.e., initially only one desktop.
Note too, that without moving to Sticky desktop, you can always popup menu and select flit thusraising it, thereby it is visible, overlaying the currently displayed app/window.