But you're not passing the hash to the service. You're passing the key. The hashing is done on a system not controlled by you, so you need to guess a plaintext

But you're not passing the hash to the service. You're passing the key. The hashing is done on a system not controlled by you, so you need to guess a plaintext that has the prefix you need. This is not an easy problem, and in fact is what is used as proof-of-work for many cryptocurrencies.