First, click upper right avatar and click update profile.
Scroll down and you will see Notifications section.
Check the webhook checkbox and you will be able to select the service.
After selecting the service, paste the webhook url in the field.
You may ask how to find webhook url?
Slack: https://api.slack.com/messaging/webhooks#create_a_webhook You will need to follow the first 3 steps and copy the url.
Gchat: https://developers.google.com/chat/how-tos/webhooks You will need to follow the instruction, copy and save the webhook url.
Mattermost: https://mattermost.com/blog/mattermost-integrations-incoming-webhooks/ You will need to follow the instruction, copy and save the webhook url.
Click the save button and you should be good to go!