Issue Getting JSON Response Page to Present on ASM Block

Given that the client-side javascript is expecting a JSON response, you need to configure ASM to provide a suitable JSON response that indicates an error in the form submission that caused blocking.

Your client-side javascript then needs to be responsible for providing a suitable message to the user that the illegal request was blocked.

There may be an existing JSON error response in the framework that you can use, or the framework may need to be modified to include a new error state. You cannot just try to insert HTML into the framework flow - the browser itself probably never sees it, just the javascript framework.