How do I make a tab effect in CSS?

Below, I would like to create the following effect on a button element in CSS:

Whereas I just want a basic regular button but with the 'tab' sticking out of the bottom.

How do I do this?

I've tried ::after and ::before, but neither even show any results for some reason (and I don't really know how to use those rules either).

.button {
  background: #aaa;
  font-size: 16px;
  color: #000;
  padding: 8px;
  width: 200px;
  border: 3px solid #000;
  border-radius: 6px;
}
.button::after {
  width: 10px;
  height: 100px;
  background: #aaa;
}

<button class="button">BUTTON</button>

You're on the right track with ::after

1. You need to include content in ::after, even if empty.

2. You have to set position: relative on the button and position: absolute on the pseudo-element so it can be placed outside the button.

3. You also didn’t tell the browser where to actually put the ::after element (like bottom, left, etc.).

example

.button {
  position: relative;
  background: #aaa;
  font-size: 16px;
  color: #000;
  padding: 8px;
  width: 200px;
  border: 3px solid #000;
  border-radius: 6px;
  text-align: center;
}

.button::after {
  content: "";
  position: absolute;
  bottom: -10px;
  left: 50%;
  transform: translateX(-50%);
  width: 80px;
  height: 7px;
  background: #aaa;
  border-left: 3px solid #000;
  border-right: 3px solid #000;
  border-bottom: 3px solid #000;
  border-radius: 0 0 6px 6px;
}