When I paste this url https://api.constantcontact.com/ws/customers in my browser and put my username and password in it doesn't work. I've tried every username and password that I have and nothing. The reason I put this url in my browser is my API Code gives my 0 :401 Unauthorized on my web page.
The URL you are trying to access is complete. Try accessing:
Also, make sure that the username that you provide in the pop-up is your api-key, followed by a '%', and then your constant contact login name like such:
Also, if you are doing this for testing purposes, I recommend using RESTClient.
Support Engineer, Constant Contact