The domain registration is 'free' for as long as you keep the hosting plan.
Each year, just before your plan anniversary, you will be given one domain registration credit to pay for your free domain. The only way (as far as I know) to tell if you have a domain credit is to actually go to the registration section of the panel and check the price shown for renewal, if you have a domain credit the price will be shown as $0.0
If you have not set your domain to auto renew, then you will need to go to the registration section of the panel and manually renew it.
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