This is not currently possible. When users first create an account, they will need to click the “Create New Account” button on the signup form.
The behaviour that you are looking for is similar to how Discourse’s implementation of SSO works. In that case, accounts are created in the background without the user filling in the signup form. It seems it should be possible to implement similar functionality for OAuth2 logins, but possibly there are cases where not enough information is passed from the OAuth2 provider to create an account.