<div class="mod-btn__wrap">
  <a href="#" rel="nofollow" class="mod-btn">Текст текст</a>
</div>

.mod-btn__wrap {
  width: 100%;
  height: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
}
.mod-btn {
  margin-top: 32px;

  position: relative;
  font-size: 16px;
  padding: 15px 40px;
  border-radius: 25px;
  background-color: #5864ff;
  color: #fff !important;
  text-decoration: none;
  border: 2px solid #2634ff;
  position: relative;
}
.mod-btn:hover {
  color: #fff;
}
.mod-btn:after {
  position: absolute;
  top: 0;
  left: 0;
  content: "";
  background-color: #5864ff;
  z-index: -1;
  width: 100%;
  height: 100%;
  border-radius: 40px;
  animation: growing 1.2s infinite;
}

@keyframes growing {
  0% {
    transform: scale(1, 1);
    opacity: 0.7;
  }
  100% {
    transform: scale(1.5, 2);
    opacity: 0;
  }
}