1👍
You should use JQuery or JavaScript. define a method in javascript and associate it with onChange event of dropdown field.
$('[name="institution"]').change(function () {
var end = this.value;
if (end == 'Other'){
$('[name="otherInstitute"]').show();
}
else {
$('[name="otherInstitute"]').hide();
}
}
Source:stackexchange.com