<div class="main">
  <div class="block">
    <p><b>Save 20% for 24 Hours ONLY! Use code JOY20</b></p>
  </div>
  <div class="block2"></div>
  <div class="block3"></div>
</div>

.block {
  position: sticky;
  top: 0;
  margin: auto;
  background-color: #2e89d5;
  text-align: center;
  color: #fff;
  padding: 5px;
}
.block2 {
  background-color: red;
  height: 500px;
}
.block3 {
  background-color: green;
  height: 500px;
}