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.

Navigate to the Feeds tab and click New Feed. In this screenshot, I'm creating it in a group I made for this guide, but it's probably easier to just create it in the Default group.

Now, make your feed. I called mine plant.

After you've created it, you should see it on your Feeds page.

Now that you've made the feed, go to the Actions tab and press New Action.

Then, select Reactive Action.

Then, select your feed in the dropdown next to if.

In the dropdown next to is, select less than.

In the text box below that, write the moisture value you want it to notify you at. I used 500, but that was really just a guess. My advice would be to wait until you normally would water your plants and then get the moisture value at that time.

In the dropdown next to Then select send a webhook message. Paste the link you got from Slack in the box under that.

When you've done that, select your feed in the dropdown to the left of value and time. 

Set the final dropdown to Slack Template, and fill in the message you'd like it to send you.