test.cs

// global flag that indicates whether the next inserted list item
// should be continued
private bool restartFlag = false;

// restarts the numbering
private void button1_Click(object sender, EventArgs e)
{
    textControl1.Selection.ListFormat.RestartNumbering = true;
    restartFlag = true;
}

// continues the following paragraph (item), if the global flag is true
private void textControl1_KeyUp(object sender, KeyEventArgs e)
{
    if (e.KeyCode != Keys.Return || restartFlag == false) return;

    if (textControl1.Selection.ListFormat.Type == TXTextControl.ListType.Numbered)
    {
        textControl1.Selection.ListFormat.RestartNumbering = false;
        restartFlag = false;
    }
}