Hi Olivier,

You can consider this thread for an example of using CSS to append additional information to the validation message. It is nontrivial to modify the error message altogether using custom code as the error message itself does not exist in the DOM until it is created. You might try the following JavaScript:

$(document).on('DOMNodeInserted', '.myCheckbox div.ws-errorbox', function() {
  $('.myCheckbox p.ws-errormessage').text('This is my custom error message.');
});

where the checkbox field is given the "myCheckbox" CSS class for selection. This may not work in older versions of Internet Explorer; see full caveats of mutation events (i.e. the DOMNodeInserted event used in the example) in this MDN article.

Note that with Forms 10.2 and onward, the ability to specify custom validation messages is available out-of-the-box.

Hope this helps.

Note: This code will not work with Forms 10.2 because of a different library for validation changing the names of certain classes; the ws-errorbox and ws-errormessage classes in particular no longer exist. However as mentioned, Forms 10.2 has its own built-in way of specifying custom validation messages.

Update: Code updated to avoid changing the message for every error message on validation error for the .myCheckbox element.