Use the placeholder for OTP in the message content as ##OTP## or if you are passing the value of OTP from your end then you need to put the same in the message as well.
In the Get method, kindly encode the ##OTP## and then pass the same in API, and in the Post method, you can directly pass ##OTP##.
Sample Get Method: http://control.msg91.com/api/sendotp.php?authkey=Yourauthkey&message=your verification code is %23%23OTP%23%23&sender=MSGIND&mobile=919999999999
Sample OTP API with OTP value: http://control.msg91.com/api/sendotp.php?authkey=youauthkey&message=your otp is 2525&sender=MSGIND&mobile=919999999999&otp=2525
For any further assistance, contact us at support@msg91.com.