At the beginning, as you have no MQTT instance, you need to create it from this page. In order to create an instance, you have to have admin role on a team.
Select your suitable plan. you may select free plan, but the free instance might no be available due to the lack of supply.
Select your suitable region.
If you select a paid plan, you will be redirected to Stripe payment page. And after the payment, you will be redirected back to DroneMQTT and automatically the instance will be setup. It will usually takes up to 3 minutes until the instance will be ready to use.