Connecting input field and button boostrap modal

I am trying to connect my input field and button while keeping the field centered on my modal. IE not floated left or right

Currently it looks like this.

I want it to look like this.

HTML---

                <div class="input-append">
                    <input id="input-zip" type="search" placeholder="Enter zip code"/>
                    <span class="input-btn">
                        <button type="button" class="btn btn-zip" data-dismiss="modal">CONTINUE</button>
                    </span>
                </div>

CSS----

.btn-zip {
    float: right;
    background: map-get($colors, bg-btn-card);
    color: map-get($colors, button-text);
    border-radius: 0;
    box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.1);
    display: table;
    margin-bottom: 10px;
}

JQUERY--

$(window).load(function()
{
    $('#zip-modal').modal('show');
});