<div class="radikalwp-fm-underline"><input type="email" placeholder="ایمیل شما" /><i><//i><//div>

.radikalwp-fm-underline { position: relative; width: 250px; }
.radikalwp-fm-underline input {
  width: 100%;
  padding: 10px 2px;
  font-family: inherit;
  font-size: 14px;
  color: #0f172a;
  background: transparent;
  border: none;
  border-bottom: 2px solid #e2e8f0;
  outline: none;
  box-sizing: border-box;
}
.radikalwp-fm-underline i {
  position: absolute;
  bottom: 0;
  right: 0;
  width: 0;
  height: 2px;
  background: #4f46e5;
  transition: width .35s ease;
}
.radikalwp-fm-underline input:focus ~ i { width: 100%; }