This is how I finally solved it. Thanks all for your help.
# on server
@anvil.server.callable
def plus10(n):
result = int(n) + 10
return str(result)
anvil.server.wait_forever()
#on client
def text_box_1_pressed_enter(self, **event_args):
"""This method is called when the user presses Enter in this text box"""
result = anvil.server.call('plus10', self.text_box_1.text)
self.text_box_2.text = result