//Form2 is the dynamically created form in your project that needs to add to a Tab
//Create the form and set it’s TopLevel property to false to enable setting its parent
Form2 frm2 = new Form2();
frm2.TopLevel = false;
//Create the new tab which will display Form2TabPageAdv newtab = new TabPageExt("FORM_2");
this.tabControlAdv1.Controls.Add(newtab);
//Set Form2’s parent to be the new tab and display the form in the newly created tab
frm2.Parent = newtab;
frm2.Visible = true;
frm2.Dock = DockStyle.Fill;