(A bug documented in the YunoHost deployment about this issue is [here](https://github.com/YunoHost-Apps/pleroma_ynh/issues/215))
Go to:
> Admin Panel > Settings > Instance
Look for the "Static dir" entry and set it to:
> /home/yunohost.app/pleroma/static
**That's it! 🎉 Soapbox is installed.** The change will take effect immediately, just refresh your browser tab. It's not necessary to restart the Pleroma service.