Let's take a walk through a tool that's pretty handy for adding links onto the portal, Link Generator. This tool will let you define a URL that can point to things outside the Application or in other systems.
As a starting point, here's the plugin for it. Its listed as Free and a part of HRSD (where I've used it) in the description, but the plugin is available stand-alone. Once I installed it in a new instance, it was put into Global scope so it seems to be independent of HRSD if you want to use it alone.
The first step is to Define a Link. The idea here is you put the base URL of whatever your link is into the Website URL field. You can give it a name as well, but that's just a description for what you're setting up.
Next you'll want to define the link parameters to be added to the ending portion of the URL through a script. This can be nothing if you want to only link to the base URL you already established, or any script or static string you want to return the back half of the URL. Choosing Button link as a Link type makes this URL available to use as a button.
Finally, use the Define Button link to add a new record that defines the config for the Button. I'm including this just to appear next to the Short Description field on all records on HR Case in Agent Workspace. You have access to the current object in the above script which in this case would apply to fields on HR Case should you need to pass in a parameter from the case record into the URL.
To test this out, I went into workspace and made sure to open a record associated with sn_hr_core_case. The Button shows up on the right side of the Short Description field and if hovered over, displays the Deep link name.
Clicking on that link brings up my search in another browser tab, tagging my search query of "servicenow link generator" to the primary URL.
