Add this css to the page to get a loader. Change the url to your loader or use theirs:

<style>
div.wpcf7 .ajax-loader {
pointer-events: none;
z-index:100;
position: fixed;
top:0;
left:0;
background:rgba(255,255,255,1);
display: block;
background-image: none;
background-image: url(/wp-content/uploads/ajax-loader.gif);
width: 100%;
height: 100%;
border: none;
padding: 0;
margin: 0;
background-repeat: no-repeat;
background-size: auto;
background-position: center;
}
</style>