Hi,
this behaviour cannot be changed.
The text is always selected when you enter a field.
If the text fields already contains a text string that should not be deleted you can use a script in the change event of that field.
It will modify the text selection coordinates so you can add additional text to your field even if the whole text is selected.
if (xfa.event.change.length > 0 && xfa.event.selStart === 0 && xfa.event.selEnd === (xfa.event.prevText.length)) {
xfa.event.selStart = xfa.event.selEnd = xfa.event.prevText.length;
}