What you described in your post is the Custom Header Links (icons) theme component. So, yes it’s an addon.
When you add links with that component, you’d have to add them like so
Title,Icon,URL,View,Target,View
I think your issue is that you’re not including the icon name for that link. Can you please try changing this
d, discord.gg/someLink, https://discord.gg/someLink, vdm, blank, keep
to this
d, fab-discord, discord.gg/someLink, https://discord.gg/someLink, vdm, blank, keep
and see if that solves your issue?