I realise that this is because both the Customer PK field and the Task FK field are set to auto-increment by 1
That's the correct setting for a primary key, but you don't define a foreign key as serial increment; it is simply to receive the value of a primary key.
and have therefore tried the alternative options but none of these seem to work.
Make it an ordinary number field without auto-entry options; if the relationship is matching Customer::customerPK to Task:customerFK, then on creating a new portal row the foreign is set automatically, i.e. you are creating a related record.