JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" >
	<input type="text" name="name" id="name" />
	<input type="checkbox" onClick="return address()" name="accept" id="accept">Accept
	<input type="text" name="cname" id="cname" />
</form>

JavaScript

function address()
	{
		var chk=document.getElementById("accept").checked;
		if(chk)
		{
		var a1=document.getElementById("name").value;
		//alert(""+a1);
		document.getElementById("cname").value=a1;
		document.getElementById("cname").disabled=true;
		}
		else
		{
			document.getElementById("cname").disabled=false;
		}
	}