This should be done automatically by formbuilder. If it isn't working, most likely it is a mismatch in the page name.
There is a bug that we change it to all lower case in one spot and not another which can cause this problem regularly when working on pages that include upper case letters. To fix it you can look for instances of the action page in the server behavior and on the rest of the page and make sure they match correctly (case sensitive).
If you want to post the server validation code and one of your form elements I can be more specific about what might need to be updated.