Python kivy how to validate TextInput with button or enter key while multiline=True?

Question

The idea is to validate a TextInput with Enter key or via a "Button"

Issue: Is there any way to run on_text_validate in TextInput: with a Button or Enter key (which also trigger the button) and down the line with shift-enter or ctrl-enter? Because I need to update the text in TextInput to my label since I can't press Enter because my multiline=True. Also is there any way to know if there are texts in TextInput, so the "validate button" will be enable and highlight when you type something in TextInput.

I tried to search on the internet but only can find 2 options, 1 is to bind keyboard, 2 is set multiline=False. I chose option1 and spent the whole day but still can not solve the issue since there are not many examples.

Edit: I added an example to make mine more clear.

.kv file

TextInput:
   multiline: True     # Down the line by hitting shift-enter/ctrl-enter instead of enter
   on_text_validate:   # I want to run this line by hitting enter or via a Button:
         root.on_text_validate(self)