How to get JWT token in back end?

In my React application I have this working authorization:

const oktaAuth = new OktaAuth({
issuer: ${process.env.REACT_APP_OKTA_ENV}/oauth2/default,
clientId: process.env.REACT_APP_OKTA_CLIENTID,
scopes: [‘openid’, ‘email’, ‘profile’, ‘groups’],
pkce: true,
redirectUri: ${window.location.origin}/${LOGIN_CALLBACK_URL},

How can I get JWT token from back-end application using same parameters (issuer, clientId, user login, user password)? I don’t have “client secret”

Usually backend makes sense when you have a traditional web-app, so your back-end will detect unauthenticated user and will redirect a browser to a login page. User will authN to Okta and Okta will return the browser to a backend callback API with an authorization code. Your backend will take it and exchange it for a token with Okta directly through an API call to /token endpoint