Just create normal form and put there some controls.
You should add there also button which will close that form. Lets say you want to add there button OK and button Cancel.
Add both those buttons there and set property DialogResult of those buttons to required value (OK and Cancel).
Then just show that dialog using ShowDialog() funtion. The function returns with the DialogResult of the button used to close the dialog.
Code:
// ....
DialogResult dlgResult = YourNewForm.ShowDialog();
if (dlgResult == DialogResult.OK)
MessageBox.Show("You close your dialog by pressing OK button");
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