Trouble Setting Up "Item Source" for AutoCompleteTextBox from WPF Extended Toolkit

I know that in the the base version of WPF, you can set up a combo box with an ItemSource for options in the drop down:

ComboBox box = new ComboBox();

box.ItemSource = List<string> exampleList;

I am using the WPF Extended Toolkit to attempt that same feature with the AutoCompleteTextBox, however there is no "Item Source" Property. I have been looking at examples online for the past few days but have yet to find an example where the suggestion list is generated. I have properly set up my XAML to include the ToolKit namespace and the actual textbox shows up fine:

<Window x:Class="complianceAuthApp.MainWindow"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

    xmlns:local="clr-namespace:complianceAuthApp"

    xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"        

    mc:Ignorable="d"

    Title="Authorization Submission App" Height="290" Width="747" Closing="Window_Closing" ResizeMode="CanMinimize">

    <grid>

        ...

           <xctk:AutoSelectTextBox 

              Name="lastNameAutoTextBox"

              AutoWordSelection="True" 

              BorderBrush="DarkGray" 

              AutoSelectBehavior="OnFocus"

              Margin="21,35,586,193"/>

    </Grid>

</Window>

However, I want to run a SQL query whenever the number of characters in the textbox reaches 3 or more:

public void getAutoTextBoxItemSource()

{



    if (lastNameAutoTextBox.Text.Length < 3)

                return;



    List<string> results = new List<string>();

    SqlConnection cnn = new SqlConnection(ConfigurationManager["SQLQuery"].ConnectionString);

    string sqlCommand = "SELECT TOP 10 ... ";



    cnn.Open();

    SQL.DataSet ds = new SQL.DataSet();

    SQL.DataTable dtable = new SQL.DataTable();

    SqlDataAdapter dscmd = new SqlDataAdapter(sqlCommand, cnn);





    int t = await Task.Run(() => dscmd.Fill(dtable));

    foreach (SQL.DataRow row in dtable.Rows)

    {

        results.Add(new string);

    }            



    //If there WERE to be an ItemSource Property...

    lastNameAutoTextBox.ItemSource=results;



    } 

 }

I can not seem to find an example that fits my needs and have spend the past few days trying to figure out how to implement my project in this way.