Within the Email Editor of a "Drive-By" or "Data Entry" Phishing Campaign, you can add or modify hyperlink(s) within the body of the email that will be sent out.
All links within a phishing email or "new campaign email" that you build will be redirected to the landing page in which you have chosen. This is true regardless if there are multiple links and if you were to add a custom link that points to another site. If you are inserting links within the body of the email itself, then you are redirecting a User to the phishing landing page domain in which you choose. You are unable to link to anything else within this email.
That said, you do have the option to send the user directly to your own URL by using the "Use Your Custom Training URL" option within the "User Training Type" dropdown. This however still sends you to the landing page domain before it redirects you to whatever you may type in to the "Use Your Custom Training URL" field.
NOTE: To add additional content, you are able to add your own links within the landing page itself. The links you customize in the "Your Custom Training Page" have the ability to send a User to whatever link you insert.