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The below CSS snippet helped me to create the shadow behing the button

form{
  isolation: isolate;
}

.button:hover::after,
.button:focus-visible::after{
  content: '';
  position: absolute;
  z-index: -1;
  inset: 0;
  background: inherit;
  border-radius: inherit;
  translate: 0 1.5rem;
  filter: blur(1rem);
  opacity: .5;
}