In the Product description toolbar, there is an option called Insert/Edit Link(chain symbol). 

To add link, follow below steps.

a) Write your word which you want to link at the required place in Product description.

b) Select the complete word.

c) Click on Insert/Edit link(chain symbol).

d) Add your link in the section provided and complete the process.

By doing this, you link gets added on the Product description section.