Remove the username

First you need to remove the @username in the hover. Paste this CSS code in your "Custome CSS" in Setup & Styling.

.bzfy-feed p {
    display: none;

Add your custom text

Write the color code you want in "color" and the text you which in "content". I select a white color and SHOP NOW. 

.bzfy-c-tilegrid-post__minimalistic-overlay:after {

content:'SHOP NOW';
color: #FFFFF;
font-size: 1em;
font-weight: bold;
text-align: center;
top: 0;
left: 0;
right: 0;
bottom: 0;
background-color: rgba(0,0,0, 0);
position: absolute;
padding-top: calc(50% - 10px);
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