Dear ashutosh.singh ,
HTTP call is basically a sync operation, as soon as you use route via http(s) to the backend, you have to wait for the response.
To async the process, or to "fire and forget", using a queue (MQ, or JMS queue) in the middle could be a solution.
The flows could be like this,
flow1
request -> your api: send notification, put request to queue, return without waiting response
flow2
queue listener -> retrieve request, route to backend, process the response, etc.
Regards,
Mark