mdb-ui-kit/scss/_input-group.scss

// -----------------------------------------
// input-group and input-group-addon styles
//  note: form-groups are not required
//
@mixin input-group-button-variation($vertical-padding) {
  .input-group-btn {
    .btn {
      //margin: 0 0 $vertical-padding 0;
    }
  }
}

// default margin - no form-group required
@include input-group-button-variation(input-padding-y);

&.bmd-form-group-sm {
  @include input-group-button-variation($input-padding-y-sm);
}

&.bmd-form-group-lg {
  @include input-group-button-variation($input-padding-y-lg);
}

.input-group { // may be in or outside of form-group

  .input-group-addon {
    display: flex;
    justify-content: center;
    align-items: center;

    background-color: transparent;
    border-color: transparent;
  }

  .input-group-addon + input,
  input + .input-group-addon {
    margin-left: .75rem;
  }
}