If you want a clickable website link within a post or on a profile on Mastodon, remember to include https:// at the beginning when you’re writing it. This tells the server that you want the link to be clickable.
Twitter automatically changes anything with a dot in the middle into a clickable link, but this doesn’t happen on the Fediverse because some people want to include dots without making clickable links.
As well as website links, you can also use many other kinds of links. All of these will work as clickable links on Mastodon: https://, http://, gemini://, dat://, dweb://, gopher://, ipfs:// and ssb://