I'm trying to import a font to my ArcGIS Hub Site using the 'Customize Theme Fonts' tool within Customize > Theme > Base Font Setting. I have the URL for Import, which is hosted on Google. This is:
https://fonts.googleapis.com/css?family=Chelsea+Market
However when I input this I get the error 'This is not a valid @import URL'. As a test I tried using the Open Sans example which is then given and I get the same error. I also get an error with the Name of the Font Family not matching the URL.

Could someone please advise what is causing the error, as it doesn't seem to accept any input at the moment.