you can use any HTTP client tool for your choice. i am using postman here

URL: http://your_domain/user/login?_format=json

Method: POST

Header: Content-Type: application/json

Body :

{
  "name": "username",
  "pass": "password"
}

Successful response:

{

    "current_user": {

        "uid": "1",

        "roles": [

            "authenticated",

            "administrator"

        ],

        "name": "admin"

    },

    "csrf_token": "Ei3oN9U_yIg8MF4NDrwX8YhsHneq7odVB3k5GhqOXWw",

    "logout_token": "xO7chKM9LaGzNLiNzSnV90QOcS6jWpmLG6Nr8TtaiTs"

}