From SMS service settings, select Clickatell and choose SMS API.

Now, follow these steps:
- Go to https://app.clickatell.com/products/channels/sms and click on Activate Channel.
- Visit the page https://app.clickatell.com/my-workspace/sms and click on New SMS setup.
- Choose API and select if you want to send One-Way or Two-Way messages and click Next.
Two-way messages: the plugin doesn’t register customer’s replies to the messages which you will be able to view only through Clickatell
- Choose Basic HTTP API and Create a new HTTP API, then click on Next.
- Complete the form with the required data and click on Next.
- Now copy and paste the generated key to the field Clickatell API Key in the plugin settings and click on Complete.
Now, follow these steps: