<div class="radikalwp-input-group">
  <input type="email" placeholder="ایمیل خود را وارد کنید">
  <button type="submit">عضویت<//button>
<//div>

.radikalwp-input-group {
  display: flex;
  width: 360px;
  max-width: 100%;
  background: #fff;
  border: 1.5px solid #e2e8f0;
  border-radius: 14px;
  padding: 5px;
  transition: border-color .25s, box-shadow .25s;
}
.radikalwp-input-group:focus-within {
  border-color: #6366f1;
  box-shadow: 0 0 0 4px rgba(99, 102, 241, .1);
}
.radikalwp-input-group input {
  flex: 1;
  min-width: 0;
  padding: 10px 14px;
  font-family: inherit;
  font-size: 14px;
  color: #0f172a;
  border: none;
  outline: none;
  background: transparent;
}
.radikalwp-input-group button {
  padding: 10px 22px;
  font-family: inherit;
  font-size: 14px;
  font-weight: bold;
  color: #fff;
  background: linear-gradient(135deg, #6366f1, #8b5cf6);
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: filter .2s;
}
.radikalwp-input-group button:hover { filter: brightness(1.08); }