The editor for composing emails in CRM provides limited functions for formatting text. In particular, there is no obvious way to insert hyperlinks.
However, you can insert a hyperlink by either writing the link in another application such as Outlook and copying the text into the CRM email or directly writing an HTML A tag as in the following example:
<a href ="http://google.com">Click here</a>
But, writing the tag only appears to work if the tag is added to an email template and not if added directly to an email whereas copying a link from another application works in an email and email template.