You’re right: you can use a Relayer directly with an API key/secret. However, keep in mind that whoever has an API key/secret for your Relayer, can freely send any txs through it - so it’s not a good idea to embed your API key/secret in your frontend, if you intend to implement meta-txs. Now, if you have your own backend, where you receive your users’ requests for forwarding meta-txs, then yes, you can have your backend interact directly with your Relayer.