Thank you for reaching out and thanks for using the Swifty WordPress Theme with BoldGrid Post and Page Builder.
You’re correct in your assumption that LinkTree is not a supported icon at the moment but there is a way to add custom social media icons your theme.
It does involve you getting your hands a bit dirty with code. You’d need to download Code Snippets and follow the directions in the guide above to add your icon.
I hope this helps! Don’t hesitate to reach out if you get stuck.