Every time i change something in .ui file from qt designer, then i compile with pyuic5 command.
Then i put MainWindow.showMaximized() instead of MainWindow.show() command.
Is there any way to do it from Qt Designer?
TL;DR; No, it is not possible.

Qt Designer is used to:

Configure the qproperties of QWidgets.
Make connections between widgets.
Edit some properties of the QActions.
Create and modify qresource.

And in none of these possibilities does it involve method showMaximized.
Maximaze window from qt designer