JSFiddle - React, Tailwind, and code Playground

HTML

<input name="name1" id="id1" value="" type="radio" >&nbsp;&nbsp;Amount 1&nbsp;$ <input name="amtname" size="5" maxlength="7" value="" onkeyup="enterAmt(this)"><br><br>
<input name="name1" id="id2" value="" type="radio" >&nbsp;&nbsp;Amount 2&nbsp;$ <input name="amtname" size="5" maxlength="7" value=""id="amt2"><br>

JavaScript

function enterAmt(ev) {
    document.getElementById('amt2').value =    ev.value;
}