how to bring the text from a textfield over to the server side? or alternative solution(read desc)

hello, so I have a mod to change the size of yourself(using pehkui), I'm usinga gui with a textfield, where you can write the size(numbers of course), and then press a button to apply it, this works in singleplayer, but not multiplayer because that "block" is client side, so is there a way to bring that value to the server, or any alternative to a textfield?