There are three ways of including links in emails:
markdown link. Use the syntax [visible text](url)
html link. Use the syntax <a href="url">visible text</a>
including the url directly
The url can be either a field value or a hardcoded url typed directly into the message body.
Issues with urls for prefilled forms
Urls for prefilled forms include the underscore character _ which is also the markdown character used to indicate italic text. This can cause problems when sending urls for prefilled forms in emails.
If you use a markdown link, the url should work as is.
If you use an html link or simply include the url, you will need to escape the underscore character.
Even if I am using a formula field to generate the prefilled url, I like to use a second formula field to escape the url. This gives me access to both the escaped and unescaped versions, which also makes it easier to maintain the unescaped version.