@Sickle-Cell Thanks again 😄
Noo, this button isn't for updating anything in the Sharepoint List, it's just to start the PowerAutomate flow that will crate an approval flow for that item.
Yes! I'm using the PowerAppsV2 trigger. Mmm I kinda get the part of the variable part as I saw it in a video tutorial, they created a String (cadena) variable that housed in its Value (Valor) a dynamic content from the PowerApps, but I only have Cuerpo (body) to add, and that did nothing. 
So what I'm lacking here?
Regarding the "PruebaPowerApp.Run(Selected.ID)" part, where would this code go?