Oauth2.0, Is it safe to authorize using post request without redirect?

So I’m trying to implement my own authorization server using spring. I have my front-end app which is implemented using ReactJs. One thing that I don’t understand is that, why do I have to redirect to the login page on the authorization server side. I want to have nice and clean login page on the client side using ReactJs without redirecting to the other site. I mean, I trust my client (ReactJs app) anyway can I just use post request to make an authentication like this.

axios.post('/oauth/login_api_without_redirect',
           { username: 'usr', 
             password: 'pwd',
             client_id: 'react_app',
             ... scope, state, etc...
             code_challenge: sha_256('yyyy'), 
             code_challenge_method: 'sha_256' })
      .then(r => {
          axios.post('/oauth/token', 
                     { ... etc params ...
                       auth_code: r.data.authorizationCode, 
                       code_verifier: 'yyyy' })
      })
   

and on the authorization server side I would have this API.

@PostMapping("/oauth/login_api_without_redirect")
public String authorize(AuthenticationDTO authen) {
    // include client_secret and other stuff..
    return authorizationCode
}

Is that safe to do so.