JSFiddle - React, Tailwind, and code Playground

HTML

<input class="defaultText" type="number" name="BetAmount" id="BetAmount" onkeyup="updateInput(this.value);">
<input type="number" name="PotentialGain" id="PotentialGain" />

JavaScript

function updateInput(ish){
    document.getElementById("PotentialGain").value = ish;
}