The promo code can be applied in two different ways.
Sign-up (Workspace creation stage)
You can apply a promo code to a workspace by clicking the link under Workspace Name when you create new workspace.
If you are an admin of the workspace, you can apply the promo code by clicking the More button next to the member.
Have you noticed?
Marimba color will be applied to workspaces with paid plans through promotions.