What the best way to test?

If you are testing you might be working in a Salesforce sandbox. Instead of using your actual Campaign Monitor account we recommend connecting a free account for basic testing.  Campaign Monitor has such a plan you can find that here - https://www.campaignmonitor.com/pricing/

And a direct sign up link here - https://www.campaignmonitor.com/signup/?sv=pricing-USD&planSelected=lite&term=annual&tier=500

If you have any questions on Campaign Monitor pricing or plans please reach out to their team - www.campaignmonitor.com