How to get status of checkbox from plugin

[bakhtiyor]

I am writing a plugin to send SMS notification to the receivers of the email. I am using a hook "message_sent" and it works fine, but I need to see if a sender wants to send sms notification to the receiver or not. For this reason I have added a checkbox with a title "Send SMS notification" in the window where user writes a message and therefore I need to check whether user wants sms notification to be sent or not.

How can I check whether that checkbox is checked or not from my plugin?

Thank you