You can send a text message to the candidate by clicking the respective button at the top of the Candidate view, as shown below:
You need to fill the following information:
Country: Select the country of the recipient's mobile number.
Mobile number: Enter the mobile number of the recipient, without the international code.
Text message: Enter the message text, up to 160 characters.
Click on the Send button to send the message, which will immediately appear in the candidate's timeline.