Hello,
Please create a widget add the html on the widget and then add the widget on the portal.
You can check the below article to see how to add widget to a page:-
https://developer.servicenow.com/dev.do#!/learn/courses/quebec/app_store_learnv2_serviceportal_quebec_service_portal/app_store_learnv2_serviceportal_quebec_service_portal_introduction/app_store_learnv2_serviceportal_quebec_add_widgets_to_a_page
Please mark answer correct /helpful based on Impact
