Pass the payload for different parameter in single request

I want to pass the payload to 2 different parameter values in single request.

For example-
Post request https://demo.testfire.net/api/login
Body:
{
“username”: “{{payload}}”,
“password”: “{{payload}}”
}

Need to know the pre-request script or test script for this scenario (sql injection).

@joycejetson @ronak26 @abhijit Can you help on this