Override the symfony 4 fosuserbundle fo security controller to implement the login check with external API

Every one. I want to override the symfony 4 fosuserbundle security controller. I am going to implement the login function with external login API(It means that When user submits login form I must check login & password in remote API). I tried this on the frontend part. But I realized that this is available on the backend part. So I am going to add the "post API and check the username and email" function on the backend part.

For this process, I override the Fosuserbundle security controller. Please let me know how can I implement this function. Thank you