error 400 - Unknown or invalid client_id

Campaign Collaborator

Hello! I'm trying to add a signup form through the plugin Gravity Forms. It's showing me an 'error 400 - Unknown or invalid client_id'. I followed the instructions (screenshots below) and have tried to troubleshoot it many times but nothing works. I'm trying to use Gravity Forms because the inline code with the universal code did not work at all either...






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Hello @KathleenP027 ,


Please reach out to the developers of Gravity Forms, as they're a 3rd party developer and we do not have access to their systems for troubleshooting.

William A
Community & Social Media Support
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Below is Gravity Forms tech support response to me regarding this. I found MailOptin ( works as a temporary alternative solution:


"Hi Joel. Constant Contact recently changed their OAuth2 authentication mechanism, and it's not possible to create new apps using the old authentication mechanism, which is still working for existing apps until March 31st. This will require a change in the add-on to use the new Constant Contact OAuth2 implementation for connecting new sites.

Our product team has this in their queue already. You'll hear from us again as soon as we have new information. Please understand that it may take longer than average to receive your next response due to the nature and complexity of this issue.

In the meantime, if you have any unused old app created (it should be one with the status "Uncopied" ), you can try changing the Redirect URI in the app to the one from your site Forms > Settings > Constant Contact page and generating a new secret.

We appreciate your patience and understanding. Thank you."

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I have the same issue which is beyond frustrating. How are you using MailOptin? I find myself needing to find a fast fix to update our Contact Us Form as well as others on our website. I noticed Elementor might be a useful plugin as well. 

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Hello KathleenP027,


Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.


Applications that integrate with Constant Contact must use the OAuth2 authorization protocol to securely authenticate a Constant Contact user account, and to be granted access to that user's data. To provide improved security, Constant Contact is now using a new authorization management service, and will be ending support for the previous service/keys on March 31, 2022.


This means that all new V3 API keys generated through the My Applications page within the Developer Portal ( will not be compatible with applications/integrations/plugins that have not yet been updated by their developers. Additionally, if the developer does not update their application, existing API keys will no longer be able to connect to Constant Contact as of March 31, 2022.


If you have any concerns about this, please contact the application's developer directly. If the developer has any questions or concerns, you’re welcome to refer them to the article link below and/or have them follow up with us directly via email at with any questions.


V3 API - Update Your Applications to Use the New Authorization Service


Support » Plugin: Gravity Forms Constant Contact


Please have a look and let us know if you have any other questions!


Courtney E.
Tier II API Support Engineer

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