Can I use a button only to change the value of a variable?

Hi all

I am a newbie at Bonita 7.2.0, and have some doubts.

I only want to change some value, but I don't know how to do it, because I have mandatory to choose one option between, send task, init process, HTTP Request, Post, Get, or Add or Delete from a collection.
I think there is no way, because now I have choosen Send task, and this changes the value, but it shows an warning. Saying Error trying to execute the task. But It makes what I want.
Other way is making this with a radio button and I think it will not show any error, but this isn't what I want to show, isn't my design.

Thank you very much.

Any assistance would be greatly appreciated.

Kind regards,
Héctor