1. First, use Postman to test the API before attempting to setup Formidable. If it works with Postman, it will work with Formidable.
2. Install the Formidable logs add-on so you can view the results of Formidable's attempt.
3. Make certain data is sent in the exact format required by the API. The most common mistake is not sending the correct date formats.

Hi,
All sorted. The issue is only related to the test connection button. That ignores the field settings. 
 
The ajax function lnked to the button is very basic. I added a content type and data type to the ajax and changed the returnerror.

function frmapi_test_connection(){
        var settings = jQuery(this).closest('.frm_single_api_settings');
        var key = settings.data('actionkey');
        var baseName = 'frm_api_action['+key+'][post_content]';
 
        var url = jQuery('input[name="'+baseName+'[url]"]').val();
        var key = jQuery('input[name="'+baseName+'[api_key]"]').val();
        var testResponse = settings.find('.frmapi_test_resp'),
                button = this;
 
        if (url == '') {
                settings.find('.frmapi_test_connection').html('Please enter a URL');
                return;
        } else if ( url.indexOf('[') != '-1' ) {
                testResponse.html('Sorry, Dynamic URLs cannot be tested');
                return;
        }
 
        button.classList.add( 'frm_loading_button' );
        testResponse.html('');
 
        jQuery.ajax({
                type:'POST',url:ajaxurl,
                contentType: "application/json",
                dataType: "json",
                data:{action:'frmapi_test_connection',url:url,key:key},
                success:function(html){
                        testResponse.html(html);
                        button.classList.remove( 'frm_loading_button' );
                }
        });
}
 
 
Regards

Matt