​link
<script src="https://getbootstrap.com/docs/4.1/dist/js/bootstrap.min.js"></script>

style
<style>
.close{
    float:right;
    font-size:1.5rem;
    font-weight:700;
    line-height:1;
    color:#000;
    text-shadow:0 1px 0 #fff;
    opacity:.5
}
.close:not(:disabled):not(.disabled){
    cursor:pointer
}
.close:not(:disabled):not(.disabled):focus,.close:not(:disabled):not(.disabled):hover{
    color:#000;
    text-decoration:none;
    opacity:.75
}
button.close{
    padding:0;
    background-color:transparent;
    border:0;
    -webkit-appearance:none
}
.modal-open{
    overflow:hidden
}
.modal-open .modal{
    overflow-x:hidden;
    overflow-y:auto
}
.modal{
    position:fixed;
    top:0;
    right:0;
    bottom:0;
    left:0;
    z-index:1050;
    display:none;
    overflow:hidden;
    outline:0
}
.modal-dialog{
    position:relative;
    width:auto;
    margin:.5rem;
    pointer-events:none
}
.modal.fade .modal-dialog{
    transition:-webkit-transform .3s ease-out;
    transition:transform .3s ease-out;
    transition:transform .3s ease-out,-webkit-transform .3s ease-out;
    -webkit-transform:translate(0,-25%);
    transform:translate(0,-25%)
}
@media screen and (prefers-reduced-motion:reduce){
    .modal.fade .modal-dialog{
        transition:none
    }
}
.modal.show .modal-dialog{
    -webkit-transform:translate(0,0);
    transform:translate(0,0)
}
.modal-dialog-centered{
    display:-ms-flexbox;
    display:flex;
    -ms-flex-align:center;
    align-items:center;
    min-height:calc(100% - (.5rem * 2))
}
.modal-dialog-centered::before{
    display:block;
    height:calc(100vh - (.5rem * 2));
    content:""
}
.modal-content{
    position:relative;
    display:-ms-flexbox;
    display:flex;
    -ms-flex-direction:column;
    flex-direction:column;
    width:100%;
    pointer-events:auto;
    background-color:#fff;
    background-clip:padding-box;
    border:1px solid rgba(0,0,0,.2);
    border-radius:.3rem;
    outline:0
}
.modal-backdrop{
    position:fixed;
    top:0;
    right:0;
    bottom:0;
    left:0;
    z-index:1040;
    background-color:#000
}
.modal-backdrop.fade{
    opacity:0
}
.modal-backdrop.show{
    opacity:.5
}
.modal-header{
    display:-ms-flexbox;
    display:flex;
    -ms-flex-align:start;
    align-items:flex-start;
    -ms-flex-pack:justify;
    justify-content:space-between;
    padding:1rem;
    border-bottom:1px solid #e9ecef;
    border-top-left-radius:.3rem;
    border-top-right-radius:.3rem
}
.modal-header .close{
    padding:1rem;
    margin:-1rem -1rem -1rem auto
}
.modal-title{
    margin-bottom:0;
    line-height:1.5
}
.modal-body{
    position:relative;
    -ms-flex:1 1 auto;
    flex:1 1 auto;
    padding:1rem
}
.modal-footer{
    display:-ms-flexbox;
    display:flex;
    -ms-flex-align:center;
    align-items:center;
    -ms-flex-pack:end;
    justify-content:flex-end;
    padding:1rem;
    border-top:1px solid #e9ecef
}
.modal-footer>:not(:first-child){
    margin-left:.25rem
}
.modal-footer>:not(:last-child){
    margin-right:.25rem
}
.modal-scrollbar-measure{
    position:absolute;
    top:-9999px;
    width:50px;
    height:50px;
    overflow:scroll
}
@media (min-width:576px){
    .modal-dialog{
        max-width:500px;
        margin:1.75rem auto
    }
    .modal-dialog-centered{
        min-height:calc(100% - (1.75rem * 2))
    }
    .modal-dialog-centered::before{
        height:calc(100vh - (1.75rem * 2))
    }
    .modal-sm{
        max-width:300px
    }
}
@media (min-width:992px){
    .modal-lg{
        max-width:800px
    }
}</style>

html
<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

js
  $('#myModal').on('shown.bs.modal', function () {
    $('#myInput').trigger('focus')
  })