private void button1_Click(object sender, EventArgs e)
    {
        string Username = textBox1.Text;
        HelloForm HF = new HelloForm();
        HF.Show();
        HF.Username = Username;
        this.Hide();
    }
}

public string Username { get; internal set; }

    private void HelloForm_Load(object sender, EventArgs e)
    {

        label1.Text = "Hello " + Username;
    }
}