How to make inline Listbox and button inside a div

Question

Hy, I'm facing some issue in CSS

 

I am not able to arrange dropdown and then button inline.

I'm using bootstrap selectpicker so if i remove selectpicker then it is working but after adding then button is comming to the next line.

 

So i want to do button should come just after listbox as inline. 

 

 

Here is my code

 

<div class="col">

<div class="input-group mb-3">

<asp:ListBox ID="ddlEmployee" SelectionMode="Multiple" runat="server" CssClass="form-control show-tick selectpicker collapse-sh" data-live-search="true" data-width="100%" data-size="5" data-actions-box="true" title="Choose one of the following...">

<asp:ListItem>John</asp:ListItem>

<asp:ListItem>Marsh</asp:ListItem>

<asp:ListItem>Tony</asp:ListItem>

</asp:ListBox><div class="input-group-append">

<button class="btn btn-outline-secondary" type="button">Add</button>

</div>

</div>

</div>

 

===

To check you need to include bootstrap 4 file and selectpicker css and js file.

 

Someone help me please i'm new in CSS. 

 

Thanks!