You can use Flock's incoming webhook to send notifications to a Flock channel from WHMCS notifications.
- Navigate to the Flock Developer Dashboard at https://dev.flock.com.
- Sign in with your Flock account and click Webhooks on the left.
- Choose to Add an Incoming Webhook.
- Select the Channel where you want incoming messages to be posted.
- Give the webhook a name and an icon that will be used to post messages in Flock.
- Click Save and Generate URL. Copy the URL that is generated.