How can I save a pattern in CC Library?

You can start, as Myra mentioned, by dragging an object filled with a pattern to the CC Library.

You can also drag multiple items filled with different patterns together into a CC library to simulate a pattern library.

The easiest way to add them to a new document is to hold down the Alt key when you drag them from the library and place them in a new document. The pattern(s) automatically get added to your swatches panel.

You can delete the dragged library items because the patterns are now part of the document.

Edit:

In the same way you can use saved gradient swatches and global colors in CC libraries.

Patterns aren't supported as an asset type in CC Libraries, but there is a work-around. Fill an object with the pattern and add that object as a graphic. Then in a new document, you can drag out the shape, double-click on it until you get down to the Clip Group (or use the eyedropper to sample it). Then the New Swatch icon at the bottom of the Swatches panel will become enabled, and you can add the pattern to your document.