Web Form rest API to add custom fields on the form

Hi Everyone,

I need to add a custom fields in all of our web-forms. I was trying to do that uinsg API. I'm able to add the standard fields in the form such as Address, State etc but when I try to add any custom field, I'm getting a 611 - System error. Am I missing something here? I'm making a call like below:

{

'method' : 'post',

'contentType': 'x-www-form-urlencoded',

'addFormFieldSetRequest':

{

'fieldId':'City'

}

}

@Sanford Whiteman, hope you can help me out here.​

Regards,
Amit