The first step of adding your dashboard to a Service Portal page is to add the Dashboard Controller widget to the page and link it to your previously created dashboard. The Dashboard Controller widget acts as the data facilitator between your instance of ServiceNow and the individual chart widgets you add to the page.
From the dashboard detail page of your previously created dashboard record, go to the settings menu by clicking the cog-wheel icon and click 'Edit Page'. This will take you to the Page Designer for the previously created Service Portal page. This assumes that you have set a Primary Page for this dashboard already in the settings menu.
In the left-hand list, search for the Dashboard Controller widget. Once you see the widget in the list, you can drag it into one of the row column layouts you have created.
Once you’ve added the Dashboard Controller widget to the layout, you’ll need to link it to the VividCharts dashboard you previously created. To do this, click the pencil icon on the top-right corner of the widget you just added.
In the Dashboard field, search for and select the name of the dashboard that you're trying to add. Once selected, click the Save button. That's it, you've now added your dashboard to your Service Portal page and it's ready to have chart widgets added.