Protecting against DDoS attack

Dear Community,

I need help from application security experts and seasoned web developers.

We are getting DDoS attacks on the following requests. This attack is targetting our SMS gateway; resulting in triggerig thousands of SMSs. Please inform which kind of protections we can introduce in application level / application code level to protect against this DDoS attack.

DDoS Request Sample:

POST xyz.com/api/otp/asdf HTTP/1.1

Host: xyz.com

Content-Length: 32

Sec-Ch-Ua: " Not A;Brand";v="99", "Chromium";v="90"

Accept: application/json, text/plain, */*

Authorization: ***********

Accept-Language: ar

Sec-Ch-Ua-Mobile: ?0

User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36

Content-Type: application/json

Origin:  http://abc.com 

Sec-Fetch-Site: same-site

Sec-Fetch-Mode: cors

Sec-Fetch-Dest: empty

Referer: http://abc.com 

Accept-Encoding: gzip, deflate

Connection: close

{"mobileNumber":"123456789"}

Warm Regards