You would have to pay any required EoG fees (if applicable) for it to be a valid mooring contract, but if you were to moor for more than 6 months you can claim a refund of the surcharge.
From the licencing portal login page: (My bold)
Evidence of home mooring requirements from 1st April 2024 (please see our FAQ page for more information)
New or existing continuous cruiser
No confirmation is required.
New customer with a home mooring
You will need to provide the details of your mooring contract, which must cover the period of your licence.
If your boat does not have a Trust index number, you will need to register prior to being able to purchase a licence. There is a charge of £20 which will be refunded if you licence within 28 days of us accepting the evidence you have provided for your mooring.
Existing customer remaining on the same home mooring
You do not need to provide the details of your mooring contract when you licence, however, we may ask you for details during your licence period.
Existing customer moving to a new home mooring
You will need to provide the details of your mooring contract,
Changing from continuous cruiser to home mooring
You will need to provide the details of your mooring contract, which must be for six months or more. If you pay for a monthly mooring, you can retrospectively claim a refund on the continuous cruiser surcharge if you can show six consecutive monthly mooring agreements/payments. In this case your status will remain as ‘continuous cruiser’ and you will receive a pro rata refund of paid surcharge from the date you notify the Trust of the change providing you have provided the necessary evidence.