focus()

js/script.js

function funActive() {
document.getElementById('txt1').focus();
}
function funInactive() {
document.getElementById('txt1').blur();
}
window.onload = function() {
document.getElementById('b1').onclick = funActive;
document.getElementById('b2').onclick = funInactive;
}

function funActive() {
document.getElementById('txt1').focus();
}
function funInactive() {
document.getElementById('txt1').blur();
}
window.onload = function() {
document.getElementById('b1').onclick = funActive;
document.getElementById('b2').onclick = funInactive;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input id="txt1" type="text" />
<br />
<br />
<input id="b1" type="button" value="active" />
  
<input id="b2" type="button" value="inactive" />
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input id="txt1" type="text" />
<br />
<br />
<input id="b1" type="button" value="active" />
  
<input id="b2" type="button" value="inactive" />
</body>
</html>