With the Hatch command active, click at a point inside the walls, as shown in the following screenshot:

You will notice that the insides of the walls are selected, and a preview will show up with the default Hatch pattern. This type of selection method is called island detection, where the Hatch command detects boundaries around the point where you click. Once you have a Hatch area selected, you can select the Hatch pattern.

All the predefined Hatch patterns are on the next panel—Pattern. Click the down-pointing arrow on the bottom right of the Pattern panel and the panel will expand to show the list of Hatch patterns:

Click on any Hatch pattern from this list. In this case, I am selecting the ANSI31 hatch pattern from the pattern list. Now click at any point inside the walls and the ANSI31 pattern will be applied in it. If the Hatch pattern does not look right within the walls, then change the scale of the Hatch pattern to increase or decrease the gap between the ANSI31 Hatch pattern lines. The scale feature is on the Properties panel of the Hatch Creation tab, as shown in the following screenshot:

Increasing the scale will increase the gap between pattern lines and decreasing it will decrease the gap. In my drawing, I will keep the scale as 1 as it looks fine on the drawing: 

Once you are done making this Hatch pattern, click the Close Hatch Creation green checkmark to apply the Hatch settings and close the Hatch Creation tab. My final Hatch pattern looked like drawing A when added with a scale of 1; drawing B is the Hatch pattern with a scale of 3.

So, now you are familiar with the way pick points work, let's look at how the Select option works in Hatch creation.