<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.13.0/css/all.css">
<button class="contact">Contact Us</button>

.contact {
  --w:600px;
  --c:(100vw - var(--w));
  
  position:fixed;
  cursor:pointer;
  border:none;
  background:#000;
  color:#fff;
  padding:clamp(10px,var(--c)*-1000,20px) 20px;
  font-size:clamp(0px,var(--c)*1000,30px);
  border-radius:
      clamp(10px,var(--c)*-1000,100px)
      clamp(10px,var(--c)*-1000,100px)
      clamp( 0px,var(--c)*-1000,100px)
      clamp( 0px,var(--c)*-1000,100px);
  font-family:sans-serif;
  transform-origin: bottom right;
  bottom: clamp(20px,var(--c)*1000,50%);
  right:clamp(0px,var(--c)*-1000,20px);
  transform: rotate(-90deg) translateX(clamp(50%,var(--c)*-1000,100%));
}
.contact:before {
  content: '\f075'; 
  font-family: "Font Awesome 5 Free";
  display:inline-block;
  font-style: normal;
  font-size:clamp(0px,var(--c)*-1000,30px);
  font-weight: 900;
  transform:rotate(90deg);
}