When you complete the health declaration on the app 24 hours before and you tick that you have tested positive you will automatically be denied boarding and there is a list of numbers to call but it was painful trying to speak to someone and it is only the sales people you get through to, not a special number.
however as you have been denied boarding on the app they will not be expecting you at the port.
as homedepot posted above you need to send your positive test via email to get the refund.
It happened to us on 26 April and still waiting for refund.