Change label's text

I have form Form1 and it have some labels. When I click on one of them, it calls new pop-up form frmPopUp. This form also has Label, which text when I click it must be inserted into label's text that called form frmPopUp. Events handlers:

        public void Label_Click(object sender, System.EventArgs e)       

         Control label = (Control)sender;           

 AddPopUpForm(); 

        public void frmPopUp_Label_Click(object sender, System.EventArgs e)        

Control label1 = (Control)sender;           

  label.Text = label1.Text; // -->?