how to get value from another from to an custom user control

Put a public event on Form1 that will take some text as an argument and update your custom control with that text.

When you display Form2 be sure to set the Owner as Form1

Then from the button click event on Form2 call the event on Form1 with the text on Form2

For example on Form1 I have placed a Button1 which displays a Form2...

private void button1_Click(object sender, EventArgs e)
{
    Form2 f2 = new Form2();
    f2.Show(this);
}

I also created an event on Form1 ...

public void AnEvent(string text)
{
    MessageBox.Show(text);   // replace this with the update to your custom control
}

On Form2 I also have a button1 that does this...

private void button1_Click(object sender, EventArgs e)
{
    Form1 f = (Form1)this.Owner;
    f.AnEvent("My text");
}