Why does my JavaScript code receive a No Access-Control-Allow-Origin header is present on the requested resource error while Postman does not

Question

I'm attempting to provide authorisation using JavaScript by connecting to the RESTful API integrated into Flask. 

However, when I make the request, I receive the following error:

XMLHttpRequest cannot load http://myApiUrl/login. 
No 'Access-Control-Allow-Origin' header is present on the requested resource. 
Origin 'null' is therefore not allowed access.

I'm attempting to provide authorization using JavaScript by connecting to the RESTful API integrated into Flask. 

However, when I make the request, I receive the following error:

$.ajax({
      type: 'POST',
      dataType: 'text',
      url: api,
      username: 'user',
      password: 'pass',
      crossDomain: true,
      xhrFields: {
        withCredentials: true,
      },
    })
      .done(function (data) {
        console.log('done');
      })
      .fail(function (xhr, textStatus, errorThrown) {
        alert(xhr.responseText);
        alert(textStatus);
      });