You must have the paint bucket selected in the toolbox, and in the popup menu in the options bar, choose Patern for Fill.

In the options bar open the pattern menu. That is the tiny triangle by the pattern preview box on the options bar.

Open the Pattern options by clicking the triangle. It's circled below.

Choose Save Patterns, and when the browse box opens, enter a unique name for the file, and save it where you can find it to load it later. Click OK.