How to resume a timer in another form?

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C#

I have Form1 and Form2.

I have a Timer in Form1, named timer.

Now, when the user clicks on the Pause button in Form1, Form2 pops up, and the timer pauses.

When the user clicks on button1 on Form2, Form2 will close and the timer is supposed to continue running, but right now I can't make it happen.

Here's what I've tried so far:

On Form1:

private void btnPause_Click(object sender, EventArgs e)
{
    timer.Stop();
    new Form2(this).ShowDialog();

}

//and a method Form2 can call to start the timer again
public void startTimer()
{
    timer.Start();
}

And on Form2:

Form f1;
Form1 form1 = new Form1();

public Form2(Form1 f)
{
    this.f1 = f;

}
private void button1_Click(object sender, EventArgs e)

{
    form1.startTimer();
    this.Close();

}

I'm referencing since there's also another button on Form2 that when clicked will close Form2 and Form1 and will show another form.
I'm not sure if I should actually instantiate Form1, but I can't seem to access startTimer() through f1.

What would be the best approach to solve this?