Unique Identifier of a multiple instance of a form

Good Day,

I'd like to ask if there is a way for me to identify or to know a some sort of unique identifier of a form instance.

Suppose I have a form and a button and from the button's click event I can create multiple instance of form2 like in this code below:

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

How will I be able to know the unique identifier of form2 even though it has multiple instance? Does it really have a unique identifier? The reason why I asked such question is that I'd like to know the unique identifier of form2, if there's any, whenever I hover my mouse or enter to its form2 instance:

private void Form2_MouseEnter(object sender, EventArgs e)
{
    //Get the unique identifier of the form that my mouse had entered.
}

I do hope you can help me guys with this. Kinda confused as to how will I be able to get the unique identifier of a single form that is being shown by creating multiple instance of it. Thank you.