If another site provided you with the html code for their banner, you can add it to your site by going to your Links tab under Marketing, selecting Link Code under the banner menu, and hitting the green Add button.
Type in a name for the banner in the Link Name field (this is only for your reference and won't show up on your site) and paste the html code in the Link Code box. Then click the green Add button when you're finished.
Add your new link code banner to a page by checking the boxes (1). Edit your link code by clicking on the pencil icon or delete it by clicking on the red X (2). Show or hide the banner on your site by clicking on the green box (3).