You can hook into the ContentCreated event and add your element when a Form newly created. Is that solving your problem?
Yeah I think it does. Do you happen to have any documentation/examples of this. For the event I guess "formsSetupCompleted" is to be hooked in an InitializationModule. How do I add the my element afterwards?
At the InitializationModule you need get the instance of IContentEvents (https://world.episerver.com/documentation/class-library/?documentId=cms/7/306eae4b-2ba2-dd1e-c114-bccb0d3d2968) by using ServiceLocator, it allows you hook into CreatedContent event. In there, you should check if the content created is your Forms then execute code for adding some initial elements into it.
Anyone have a prefered way of adding a checkbox (which has validaiton on it to be checked) to all instances forms?
Extend and create a new form and change the template to include checkbox + validation?
Create a block element and always add this somehow to all instances of a specific form type?
Just help user by somehow always adding it to the element list (still removable)?