how to set a form as a child form to an MdiParent?

I have an MdiParent form and several child forms that open inside the MdiParent.

But within one of the child forms(lets call it Form1) I have a button that when clicked it opens another form(lets call Form2). The problem is that when I click the button on Form1 it opens Form2 but Form2 is not a child of the MdiParent. I can move Form2 outside the MdiParent borders.

I've tried to set Form2.parent = MdiForm but that does not work.

What am I doing wrong?

Thank you