function maybePrefixUrlField() {
	if(this.value.trim() !== '' && this.value.indexOf('http') !== 0) {
		this.value = "http://" + this.value;
	}
}

var urlFields = document.querySelectorAll('.mc4wp-form input[type="url"]');
if( urlFields && urlFields.length > 0 ) {
	for( var j=0; j < urlFields.length; j++ ) {
		addEventListener(urlFields[j],'blur',maybePrefixUrlField);
	}
}