I have custom SharePoint list with several fields. Two of the fields are TargetPage and ExternalURL. At the moment the user is allowed to fill out both of them if he want to, and I want to deny that. How can I "validate" the form before it's submitted so that the user is only allowed to fill out one field at a time?
A simple solution is to just validate the fields on submit by adding a validation rule in the list settings.
In the formula add your formula, I think it will be like this in your scenario:
And add a user message in the list input field.