I want the value user enter in the Prompt shows in the input text field id="myName"
var a = prompt("Enter your name");
document.getElementById("myName").innerHTML.value = a;
<input type="text" id="myName">
Don't use innerHTML here, just change the property value.
var a = prompt("Enter your name");
document.getElementById("myName").value = a;
<input type="text" id="myName">