Dear Samuli: Thank you so much for taking the time to help me!
I tried your code in method FPM_SETUP_APPLICATION, but nothing changed in the webdynpro when I tried the service. I checked again the configuration application and that button does not have anything assigned in the field "element ID", could it be the reason why I can not reference it using iv_element_id?
It is not a big issue though, with the change of the text of the other button I'm more than done
Thanks again !