Step 1: Create a free test account here
Step 2: Go to API's tab here
Step 3: Select #4 Beta API here
Step 4: Scroll down and click blue button "Open Console"
Step 5: Add a message in the message box
Step 6: Do not enter any headers and then click button "HTTP GET"
Step 7: Review error message returned from wolfram
Step 8: Go back and add this header into the headers box "User-Agent:Mozilla"
Step 9: Click "HTTP GET" again and see that it returns 200 and works fine