Disable autofill Keychain of safari without disable autofill third party app

Hi,

I'm using third party app of password manager (StrongBox) on the Mac (Big Sur 11.3.1).

When I get the login page in safari, I get in the options the iCloud Keychain of safari and the StrongBox. Also after I insert the password from StrongBox, the Safari asks me if save the password in the Keychain.

How can I cancel the keychain option but to stay the strongbox autofill? 

I already disabled the keychain in the Apple ID iCloud and if I disable autofill of password through the preferences of safari, it will also disable the StrongBox autofill.

Thanks, 

Michael