Please check the aspx page code as below =
<div>
<asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown"></asp:textbox>
<asp:button id="Button1" runat="server" text="Button" onclick="Button1_Click" xmlns:asp="#unknown" />
<asp:dropdownlist id="DropDownList1" runat="server" datasourceid="SqlDataSource1" datatextfield="Name" datavaluefield="Id" xmlns:asp="#unknown"></asp:dropdownlist>
<asp:sqldatasource runat="server" id="SqlDataSource1" connectionstring="<%$ ConnectionStrings:LOC_MASTERConnectionString %>" selectcommand="SELECT [Id], [Name] FROM [Fruit]" xmlns:asp="#unknown">
</asp:sqldatasource>
</div>
Please check the code behind =
protected void Button1_Click(object sender, EventArgs e)
{
DropDownList1.SelectedValue = TextBox1.Text;
}