Hi Julien,
I wasn't able to get that running through the Rules Manager.
However, you can use the 'Run Script' option instead to run a piece of JavaScript that can handle that for you: "When CrmTextField_C005 changes () then execute field setTextBox() (with )"
Here's the setTextBox function that I used to set the text content of one text box to another: setControlValue(findControl("CrmTextField_C004"), findControl("CrmTextField_C005").get_value());
You can also do it like this: findControl("CrmTextField_C004").set_value(findControl("CrmTextField_C005").get_value());
Additionally, you can find references to the client side API
here.
Please let us know if you have any more comments or concerns.
Kind Regards,
Chris Rooney.