Make sure that you have the SMS feature enabled, and enough credit to use.

in the form (once you've filled out the fields that you would like to be done) you can select Convert to Webform under the Title, and Save. Then you will have more options appear.

Select Copy Link, and then go to CRM > Contacts > select contact > Communication > Send > SMS. You can then paste the webform link in the SMS body and send. The client will receive the link to the webform which they can then complete.

Alternatively, select the Quick Send Icon form the Top menu bar > Select SMS > Paste the Webform link from the Form into the message.