If you are ever unsure of the correct action: ask customer support.
It may simply be that you missed the coupon field, for instance, it may be further down the page or hidden in plain sight. It may also be that the casino has a different set-up, such as having to give the code to customer support. If you are already speaking to support you will thus be well on the way.
You can also confirm with our casino reviews. We test each new casino with real deposits and usually claim the welcome bonus, as well as any possible registration bonus. This means we probably have encountered the same issue you did.