Hi Vítor,
We are happy to announce that your idea has been implemented and made available with version 11.6.1 (Build 3701) of Service Studio.
Below you will find a short description of this capability, from our "Reactive Web: The Next Generation of Web Apps" forum post. So you can see that, besides having this for Mobile Apps, Reactive Web Apps also benefit from the same exact experience.
Default Buttons

"Button widgets in mobile and reactive web apps now have the Is Form Default property, which you can set to Yes and have the App submit the data from the related form when the end-user presses the Enter key."
Thanks again for your idea!
Cheers!