Can I have both the options thank you page as well as redirect to external link?

I understand that you want to use both options to display the Thank you message and then redirect to an external link. Unfortunately, it is not possible to enable both the options on a form. You may setup conditions to display a message or to redirect to a web page. 

https://www.jotform.com/help/317-Change-Thank-You-Page-URL-Based-on-a-Condition

https://www.jotform.com/help/372-How-to-Change-Thank-You-Message-Using-Conditional-Logic

Thanks!