If you're posting a link on Mastodon, you'll probably want to add https:// to the beginning so it is clickable.
For example fedi.tips isn't clickable but https://fedi.tips is clickable.
The reason https://is required is in case people want to include dots in words or phrases without accidentally creating links.
p.s. Techy people might like to know that many non-web link types also work on here: https://, http://, gemini://, dat://, dweb://, gopher://, ipfs:// ssb://