I searched everywhere in the Discourse icon library but couldn’t find this icon:
Since the 
seedling icon is not part of the default list of icons, you have to add it to the SVG icon subset site setting before it’s available to use 
The regular (far-) version of the icon is a pro icon. You won’t be able to use that without Discourse FontAwesome Pro
But the screenshot in your first post shows the solid version. That one should work. But you add it without a prefix
Additionally, I’m not sure if this is a bug, but although I added it to the Font Awesome SVG list in the admin backend, it disappeared after refreshing.
Looks like I accidentally typed an extra prefix. I’m such an idiot; what should have taken just a few minutes ended up wasting nearly an hour (just like Chen Qianyu from Endless)……
Do you know why this is needed now? I think a while ago it was possible to select new icons for badges without adding them to the site setting (with a quick search, I only found a PR adding the parameter to only show available icons in the icon picker for sidebar links, because users aren’t able to add new icons, which indicates that in other places this was possible).
Discourse offered all the icons available on FontAwesome in search, which is really helpful because otherwise there is no way to learn about the icons in Discourse. It doesn’t seem to match the strategy of making things easier to set up if you now need to search on FontAwesome.com, add the icon to a site setting, and add it to a badge instead of searching and selecting it in the icon picker.
And in this case it would have helped avoid the struggle of using the correct prefix.