private void 用户名文本框_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == '\r')
    {
        this.密码文本框.Focus();
    }
}

private void 密码文本框_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == '\r')
    {
        this.登录按钮.Focus();
    }
}