$.ajax({
    method: "POST",
    url: "/user/login?_format=json",
    dataType: "json",
    contentType: "application/json",
    data: { user: "myname", pass: "mypass" }
})

user.login:
  path: '/user/login'
  defaults:
    _form: 'DrupaluserFormUserLoginForm'
    _title: 'Log in'
  requirements:
    _user_is_logged_in: 'FALSE'
  options:
    _maintenance_access: TRUE