you will need to replace the Lightweight Social Icons plugin as it cannot have new icons added.
Instead if you’re using the Block Widget editor in Dashboard > Appearance > Widgets then you can create your own social links using the GenerateBlocks Buttons block:
We have read all the posts on adding Tiktok. All deal with using Blocks as Lightweight Social Icons won’t be updated.
We still haven’t changed over (too many employees working on the site). We still want to add Tiktok, which has now become the most important social media site.
Is there another way to add the icon/link while using the classic editor?
Our primary suggestion would be to use GenerateBlocks Buttons Block as mentioned by David.
However, since you’re using a classic editor, perhaps it would be good to try a different plugin instead. For instance, Fontawesome plugin. I believe this can be used in the Classic editor.