It seems that it is a property of the input text component from React Native:
https://reactnative.dev/docs/textinput#clearbuttonmode
I’m just not sure whether this can be changed using the default Mendix text input widget.
You may be able to style it differently, i’m just not sure what the default class for it would be.
Otherwise you could build a simple widget with the property set to not show this clear option.