You have three options to do that:

1. Go to Candidates, click the three dots by a selected candidate and choose Send SMS. If the telephone number is missing from the profile, you will have to enter it manually:

2. Open a selected candidate profile, hover over the red circle at the bottom right corner of the screen and choose Send SMS:

3. To multiple candidates at a time. To do that, go to Candidates, select the candidates you want to contact and select Send SMS from the menu at the top: