"Plain text" literally means "plain text." You can't get any pretty formatting in a plain text email. What you send is what the recipient receives. Instead, you need to use HTML format instead.
You need to use HTML tags, as described in How do I add a link to my email signature?.
Go to: Setup | Personal Setup | Email | My Email Settings
- Enter the path to the website in the Signature box enclosed in HTML tags as follows
Enter words that display for link
<a href="http://login.salesforce.com">Link to Salesforce
NOTE If the link does not show, type the coding into the box rather than copying/pasting as sometimes extra characters can get
attached with the cut and paste process. When sending an email from
Salesforce, you need to make sure the emails are in HTML format. On
the web page to write an email (after buttons "send an email"), locate
the section "Email Format". It's at the top of the page, on the right
near the "To" field.
If it says "HTML", you are ok.
If it says "Text-Only", you need to click on "Switch to HTML".
If you send an email in "Text-Only" with the modified signature, the
logo will not appear, and the signature will include as Enter words that display for link here
instead of the highlighted words that click to the website.