Click the ⌄ button just to the right of the ⊕ Add to map button.
Choose Search for objects that intersect a shape...
In the blue toolbar that appears across the top, choose the Circle shape option.
Click into the center of the SEA airport and drag your circle out to ~10 mi or ~20 km (you can change your default units by clicking the settings "cog" icon in the upper-right of the application screen near the Save button). A search window will open when you click to confirm the radius (or click "Done").
In the search window, click the [Example Data] Airport object type in the Recently Used section. The only results that return are those that fit within the drawn shape.
Click the SEA result and then on the blue ⊕ Add selected button in the bottom right of the search window.
Click the blue Save button in the upper-right corner of your application screen.