Getting 400 bad request while submitting response from a custom HTML page through power automate

Hi Shukla, Atul {PEP}, 

Welcome to Microsoft Q&A Forum! Have a good day and I hope you're doing well! 

Thank you for sharing the detailed information and screenshots. I understand how frustrating this must be, especially when the survey loads correctly but fails with a 400 Bad Request right after clicking Submit. 

Based on my research and testing in a similar setup, a 400 error in this scenario is typically caused by a schema validation mismatch in the HTTP triggered Power Automate flow. 

When a flow uses the trigger “When an HTTP request is received” with a defined JSON schema, Power Automate strictly validates the incoming payload before the flow actually runs. If the submitted data does not exactly match the expected schema, for example a field defined as an integer is received as a string such as "305" instead of 305, the platform immediately returns a 400 response. 

One common cause in custom HTML forms is that JavaScript input.value always returns a string, even if the value looks numeric. If the flow schema expects an integer, this mismatch alone is enough to trigger a 400 error, and the flow run may not even appear in the run history because validation fails at the trigger level. 

This behavior aligns closely with what you are experiencing: 

• The form loads successfully 
• The POST request is sent 
• The request fails immediately with 400 

This strongly suggests a payload and schema mismatch rather than a connectivity or authentication issue. 

Please note that this forum is not the official Power Automate platform forum, but I will do my best to assist you here. If you need deeper or more platform specific Power Automate guidance, you may also consider posting on the Power Automate Community Forum

Additionally, if possible, you may try creating a support ticket in the Power Platform Admin Center:  https://admin.powerplatform.microsoft.com/support/requests 

Please note that to create a support ticket in the Power Platform Admin Center, you must have the appropriate administrative role or permissions. For your convenience, here is the official Microsoft guidance on how to request support: Get Support 

In the meantime, I will continue testing this scenario in a controlled environment to better simulate your setup and confirm the exact behavior. I may need some additional time to validate everything thoroughly, but I will make every effort to provide more concrete findings. 

If you have any updates, such as additional error details, screenshots, or changes in behavior, please feel free to share them. That will help us gain further insight and narrow down the cause more precisely. 

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