Ctrl‑drag (Windows) or Command‑drag (Mac OS) the color from the Tools panel or Color panel to the Swatches panel.
In the Swatches panel, Ctrl‑click (Windows) or Command‑click (Mac OS) the New Swatch button, or select New Swatch from the panel menu. In the dialog box that appears, select Spot Color for Color Type. Set additional swatch options and click OK. (See Swatch options.)