How do I add hatching to a block in AutoCAD?

You need to add a pattern to the selected object.

You start by selecting the options you wish to apply and then select the object and apply.

1.  First, select Hatch, located at: Home Tab > Draw Panel > Hatch.

2.  Choose the Hatch you want in the Properties panel > Hatch Type list,.

3. Select whether you want a hatch pattern or fill in the Pattern panel.

4. Now select the boundary option in the Boundaries panel. In your case you’ll probably need ‘Select Boundary Objects.’ meaning it will apply to any closed object selected.

5. Select the object you wish to hatch an area or object to be hatched.

6. Hit Enter on your keyboard to apply the hatching to the selected object.

Source: To Work With Hatching or Filling Objects or Areas (AutoDesk Knowledge Network)

Here’s some related videos

Place the mouse on the block, right button, edit block in place, OK, you apply the shading to the block,
and save the shaded block leaving the block editor.
finish.

If you want to apply a hatch on all instances of the block, you’ll have to edit the block itself. If you only want to make a hatch to one specific instance, add it how you normally add hatches.