Hi @Alex_Wu, welcome!
You should be able to do this now using Anonymous Authentication. For example using the JavaScript SDK:
Stitch.defaultAppClient.auth.loginWithCredential(new AnonymousCredential()).then(user => {
Stitch.defaultAppClient.callFunction("foobar", [argument]).then(response=>{
console.log(result);
});
}).catch(console.error);
Regards,
Wan