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Advocate IV
Advocate IV

CORS failed to fetch error with custom HTTP connector with Basic auth

Trying to access a 3rd party REST API. It fails with the built-in HTTP connector. Thus creating my own connector and it still fails. Here is the screenshot showing you the def of the connector in the Swagger editor with the curl and result to the right.  Could all this be due to Microsoft not having properly initialized the JavaScript fetch() API credentials options like so:


'To send credentials in fetch, we need to add the option credentials: "include", like this:'

fetch('', { credentials: "include" });



Advocate IV
Advocate IV

FYI this curl works as intended. So what is the Microsoft JavaScript doing under the covers???

Advocate IV
Advocate IV

For those searching, I found out why all this nonsense behavior is happening... but I'll wait before posting the reason until Microsoft gets off theirs a$$es and actually provides some well needed support re this. Why?  Because this concerns one of the most basic user interaction with the UI and it should have been explained and highlighted and documented wide and clear with an easy to find link so that a basic search would find it, all of which are not the case today.  And that is unacceptable considering the amount of time wasted at guessing why (hint) what looks like an editor textbox does not actually behave like an editor.

Please @iso , please share the solution. I've been searching for 2 days and no luck. Much appreciated!

Fine... Microsoft should be paying us for doing all the beta testing and providing support to each other. 


1_ if the GET request uses the header Content-Type: application/x-www-form-urlencoded, then you have to URL encode manually the following fields (assuming this is the URL to GET the token):  app key, app secret, and other request params that are joined by '&' , in other words, in the curl screenshot above, everything that follows --data-urlencode, you should URL encode manually using the encodeUriComponent() function like so:


add a Compose action to your flow and type in the Expressions tab that shows up in the dialog to the side :  encodeUriComponent( 'app_key' ), where 'app_key' is a string of characters typically provided by the REST API author; so if you have 4 such params, then you would need 4 Compose actions, and so on.  If using variables for app_key etc instead of typing the string literal, then make sure you click on Dynamic content and select the variable in the list that contains the value you want to pass to the encodeUriComponent() function.  The flow UI is picky when it comes to typing everything straigth into an action textbox versus using the Dynamic context and/or Expression tabs in the edit dialog that pops up after clicking inside a textbox.


2_ the textboxes you see in the UI for the HTTP action apparently are not meant to be used as if you were in an editor; for ex, to enter the Body of the request, if you type in app_key=encodeUriComponent('your_app_key')&app_secret=encodeUriComponent('your_app_secret')&.... and so on, it won't work; apparently, Power Automate Cloud expects you to type the first half, ie 'app_key=' and then go to the Dynamic Content tab of the dialog that opens up when you click inside a textbox and from there scroll down until you find the output of your previous compose actions and add these outputs using the GUI; at least for me, my first reflex was to type everything in and it simply refused to work.


http_post_token_req_Screenshot 2022-05-04 142030.png


And that's it. As you can see from the screenshot above, once you click on each Compose output, it appears as a purple widget inside the textbox. 

Thank you VERY much @iso . Worked in a flow for me so that's awesome. Still figuring out how to get it to work in a custom connector though. Thanks again!!

You're welcome. Please click the 'Accept as Solution' button.  😁

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