Google Travel Time - restricting credentials

I don’t know if this the right place to ask but…

Can anyone tell me what application restrictions I need to apply to my Google Maps Distance Matrix API?

I don’t have a fixed IP so I am using duckdns and have tried just about every possible way of representing

that that I can think of in the API key credentials page on Google. I am assuming that I need to restrict ‘HTTP referrers (websites)’ but of course I might be looking down completely the wrong path!

Any help would be much appreciated. Thank you.