Bootstrap class: .input-group-append

<div class="input-group">
  <input type="text" class="form-control" placeholder="" aria-label="" aria-describedby="basic-addon1">
  <div class="input-group-append">
    <button class="btn btn-success" type="button">Love it</button>
  </div>
</div>

Preview

Sass source

/* _input-group.scss:69 */
.input-group-append {
  display: flex;

  // Ensure buttons are always above inputs for more visually pleasing borders.
  // This isn't needed for `.input-group-text` since it shares the same border-color
  // as our inputs.
  .btn {
    position: relative;
    z-index: 2;
  }

  .btn + .btn,
  .btn + .input-group-text,
  .input-group-text + .input-group-text,
  .input-group-text + .btn {
    margin-left: -$input-border-width;
  }
}

/* _input-group.scss:89 */
.input-group-append { margin-left: -$input-border-width; }


// Textual addons
//
// Serves as a catch-all element for any text or radio/checkbox input you wish
// to prepend or append to an input.

.input-group-text {
  display: flex;
  align-items: center;
  padding: $input-padding-y $input-padding-x;
  margin-bottom: 0; // Allow use of <label> elements by overriding our default margin-bottom
  font-size: $font-size-base; // Match inputs
  font-weight: $font-weight-normal;
  line-height: $input-line-height;
  color: $input-group-addon-color;
  text-align: center;
  white-space: nowrap;
  background-color: $input-group-addon-bg;
  border: $input-border-width solid $input-group-addon-border-color;
  @include border-radius($input-border-radius);

  // Nuke default margins from checkboxes and radios to vertically center within.
  input[type="radio"],
  input[type="checkbox"] {
    margin-top: 0;
  }
}

Uses variables

Code copied to the clipboard.

Copying failed

/* _input-group.scss:69 */
$input-border-width

/* _input-group.scss:89 */
$input-border-width
$input-padding-y
$input-padding-x
$font-size-base
$font-weight-normal
$input-line-height
$input-group-addon-color
$input-group-addon-bg
$input-border-width
$input-group-addon-border-color
$input-border-radius

More in Bootstrap Form Input Groups