Here is some CSS and javascript that we use on our site for the popup. You can either add these to your theme or use a plugin such as Scripts n Style to add the css and javascript to your own site.
You can review this blog article on different ways to add javascript to your own site 3 ways to insert JavaScript into WordPress pages or posts.
Also keep in mind that each theme is a little different and you may need to adjust the javascript and css to work for your site, but this should get you started.
css code
/**-------------------
*SEARCH POPUP
*--------------------**/
#blackout {
width: 100%;
height: 100%;
background-color: rgba(0,0,0,.75);
z-index: 9999;
position: fixed;
top: 0;
display: none;
}
#closePop {
float: right;
height: 25px;
width: 25px;
position: absolute;
right: 0px;
top: 0px;
color: red;
font-size: 25px;
cursor: pointer;
}
#popResults {
width: 60vw;
height: auto;
position: fixed;
top: 180px;
left: 20vw;
background-color: rgba(255, 255, 255, 0.75);
z-index: 99999;
padding: 20px;
min-width: 345px;
display: none;
}
#popResults .domain-result {
float: right;
vertical-align: middle;
padding: 10px 10px 0px 8px;
width: 100%;
min-width: 290px;
border: 1px solid #e8e8e8;
}
.salePrice {
font-size: 20px;
font-family: Oswald;
text-transform: uppercase;
color: #2eb343;
}
.domain-name {
font-size: 20px;
font-family: Oswald;
text-transform: none;
color: #353535;
float:left;
}
.purchase-info {
float: right;
}
a.rstore-domain-buy-button.submit.button.select {
margin-left:15px;
}
.rstore-disclaimer {
clear: both;
}
.rstore-disclaimer pre {
font-family: sans-serif;
background: none;
border: none;
font-size: 10px;
font-weight: 100;
margin: 0px;
padding: 0px;
white-space: pre-wrap;
}
javascript code
jQuery(document).ready(function($) {
// build popup on body
$('body').append($('<div/>').attr("id", "popResults").addClass(""));
$('body').append($('<div/>').attr("id", "blackout").addClass(""));
$('#popResults').append($('<div/>').attr("id", "closePop").addClass("").html("X"));
//move domain search results on click
$('.rstore-domain-search').on('click', '.rstore-domain-search-button', function(){
$('#blackout').fadeIn();
$('#popResults').fadeIn();
$('.result-content').fadeIn();
$('#popResults').append($('.result-content'));
});
//remove search results and add the results div back to the search bar
$('body').on('click', '#closePop, #blackout', function(){
$('#popResults').fadeOut();
$('#blackout').fadeOut();
$('.result-content').hide();
$('.search-box').append($('.result-content'));
});
});