Not having seen the page, I’ll guess this text is to be on the standard RetroTube Categories page?
It’s quite difficult to insert a piece of text accurately on that page, but you can make use of the text allowed in Videos -> Video categories.
With some SEO plugins (Yoast) and themes, you can edit that description to include HTML. In ‘All In One SEO’, or the standard RetroTube theme, you can’t, so you’ll have to use a plugin like ‘Visual Term Description Editor’ to get the link formatted correctly.
I’ll include both the HTML and text to insert as I’m not sure how this editor will treat HTML:
<p>why watch videos when you could be watching <a href="yourdomain.com/yourlink/"> live cams</a></p>
<p>why watch videos when you could be watching < a href="your domain.com/ yourlink/"> live cams</p>
Or you could put that in a text box on the side bar.
You can use a text box to include dynamic links to advertising or your own links, much easier directly on the home and video pages, footer and side bars, where you have a lot more play to include whatever you want.