URL to an external document in a form

Document ID : KB000115177
Last Modified Date : 17/09/2018
Show Technical Document Details
How can I create a link to an external document in my form?
There is an example in the out of box form "Operational Effectiveness Form" under the "Service Management Dashboards Forms" group.
You can have external content with something like: 
<iframe id='reportContainer' height='100%' width='100%' src="JavaScript:' '"></iframe> 
Or alternatively you could include a simple URL link with an HREF tag.