.t-btn {
  position: relative;
  overflow: hidden;
  border-radius: 40px; /* закругление, как у твоей кнопки */
}
.t-btn::before {
  content: '';
  position: absolute;
  top: -30%;
  left: -40%;
  width: 70%;
  height: 160%;
  background: linear-gradient(120deg, rgba(255,255,255,0.20) 0%, rgba(255,255,255,0.55) 60%, rgba(255,255,255,0.20) 100%);
  border-radius: 40px; /* скругление */
  transform: skewX(-20deg);
  animation: shine 2.7s infinite;
  pointer-events: none;
}
@keyframes shine {
  0% { left: -40%; }
  100% { left: 120%; }
}