Normally, you can log in with the credentials you've created after purchasing the plan. So, most clients don't have to worry about this.
However, if you didn't create an account because you closed the purchase window early, you should look for an email in your inbox where we invite you to do so.
You should look for this email subject line: Payment Processed – Create Your Account. If you don't see this email, make sure you're checking the correct email in case you use several. Also, take a look at your Spam and Promotions folders. Sometimes important emails end up there by mistake.
Once you follow the steps in the email, you'll be able to log in to the app and use it.