Open my page in IFRAME from main menu

Hi,

I'm completely new to both this forum and to Dynamics, so please forgive me if these are stupid questions.  My goal is to load my external site in an IFRAME within Dynamics 2016 so that the masthead remains visible, but my IFRAME takes up the rest of the screen.  I created a new Entity and set it up as part of the Sales area, and now it appears as a link in the Extensions area under Sales.  So far so good.  When I click that link, however, it takes me to the basic landing page which just has a grid list.  Is there a way to customize this landing page?  I am able to customize a Main Information form to add an IFRAME and manipulate it with Javascript from there, but that's not getting me where I need to go.  Is this the right approach?  Any other ideas?

Best regards and thank you in advance,

Jim