Hello, guys! Regards from Brasil 
It’s possible to create a new feature using a button in the map canvas, without selecting the layer and activating the edition? There’s any method to customize QField for it? Thanks!
Hi!
Would this plugin be close to what you want to achieve?
If you disable all the toggles in the plugin settings, you basically get a button that just creates a point feature on the current layer at your current position, without the need to enter digitize mode.
If you were to develop something similar, you could add a layer setting, which should allow you to create the feature on your predefined layer, no matter which one you have currently selected.