JSFiddle - React, Tailwind, and code Playground

HTML

<div class="fancy-box-with-button">
  <input type="text"/>
  <button>Submit</button>
</div>

CSS

.fancy-box-with-button {
  background-color: white;
  border-radius: 4px;
  margin: 10px;
  display: block;
  position: relative;
  height: 30px;
  box-sizing: border-box;
  padding-right: 100px;
}

.fancy-box-with-button input {
  height: 100%;
  width: 100%;
  border: none;
  margin: 0;
  background-color: transparent;
}

.fancy-box-with-button button {
  position: absolute;
  top: 0;
  right: 0;
  margin: 0;
  height: 100%;
  width: 100px;
  border: none;
  color: white;
  background-color: blue;
}