how to write connection string automatically in web.config f

Question

How to use this form for creating an connection string in the web.config file on the press of "Submit & Continue" button. As I want to develop an application where any user can create a database connection just by entering database type, URL and authentication details. Please help me in all possible ways.

<div class="box box-info box-solid">
        <div class="box-header with-border">
            <h3 class="box-title">Add Database to Your Store</h3>
        </div>
        <div class="box-body">
            <label class="text-info text-bold">First Create a Database on Your Server. If You need help in creating database on server then <a href="#" target="_blank">Click Here</a></label><br /><br /><br />
            <label class="col-sm-3 col-md-3 col-lg-3">Database Type : </label>
            <div class="col-md-9 col-sm-9 col-lg-9" style="margin-bottom:10px;">
                <asp:DropDownList ID="LanguageDropdown" runat="server" CssClass="form-control">
                    <asp:ListItem>MS SQL Server</asp:ListItem>
                    <asp:ListItem>MY SQL</asp:ListItem>
                    <asp:ListItem>Oracle DB</asp:ListItem>
                </asp:DropDownList>
            </div>

            <label class="col-sm-3 col-md-3 col-lg-3">Host : </label>
            <div class="col-md-9 col-sm-9 col-lg-9" style="margin-bottom:10px;">
                <asp:TextBox ID="DatabaseHost" runat="server" CssClass="form-control" placeholder="Write 'localhost' if Database Server & Web Server are same."></asp:TextBox>
            </div>

            <label class="col-sm-3 col-md-3 col-lg-3">Database Name : </label>
            <div class="col-md-9 col-sm-9 col-lg-9" style="margin-bottom:10px;">
                <asp:TextBox ID="TextBox1" runat="server" CssClass="form-control"></asp:TextBox>
            </div>

            <label class="col-sm-3 col-md-3 col-lg-3">User Name : </label>
            <div class="col-md-9 col-sm-9 col-lg-9" style="margin-bottom:10px;">
                <asp:TextBox ID="TextBox2" runat="server" CssClass="form-control"></asp:TextBox>
            </div>

            <label class="col-sm-3 col-md-3 col-lg-3">Password : </label>
            <div class="col-md-9 col-sm-9 col-lg-9" style="margin-bottom:10px;">
                <asp:TextBox ID="TextBox3" runat="server" CssClass="form-control" TextMode="Password"></asp:TextBox>
            </div>
        </div>
        <div class="box-footer">
            <asp:Button ID="Next" runat="server" Text="Submit & Continue" CssClass="btn btn-success pull-right" OnClick="Next_Click"  />
        </div>
    </div>