@Sangeetha Priya S Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
You can refer to the sample example mentioned in the documentation, on how to send 1:1 message using C# and using alphanumeric sender.
Note: You need to use your alphanumeric sender number in the from-phone-number value in the above screenshot.
Feel free to reach back to me if you have any further questions on this.