First click on the name of the workspace, then Settings & administration (or just Administration is you're not an admin) and then Manage apps. If you're not an administrator, you should see less options on the last menu but the one you need should still be there.

Now click on Custom integrations.

After that, type Incoming WebHooks into the search bar and click on the first result.

Then, click Add to Slack.

You should now see a screen that looks something like this. Here you'll decide which channel you want the webhook to post into. For this example, I'm using a direct message to myself, so I'll select my name from the list.

Finally, you should see this screen. Copy the Webhook URL (the one I've put a red rectangle around) and paste it somewhere so you have it for the next section.