Can't change field value from custom button

Can't change field value from custom button

Hi,
I have been trying to get a custom button in my stateless form to change one ore more field values in the form. Using the syntax input.SomeField = "SomeValue"; works fine when used in onLoad actions or any of the field actions, but it does not seem to do anything in an on Click action?
Ist this deliberate? Is there a workaround? Am I missing something?

Thanks in advance!