Edit request body in Pre-request script

New to APIs/Postman
18

You have to set variable placeholders in the body itself, then set the variable values in the pre-request script. So you request body will be:

{
    mode: 'raw',
    raw: JSON.stringify({
        licenseUsername: '{{username}}',
        licensePassword: '{{password}}'
    }),
    options: {
        raw: {
            language: 'json'
        }
    }
}

…and in you pre-request script:

pm.variables.set("licenseUsername", "username-value");
pm.variables.set("licensePassword", "password-value");

This allows the flexibility of changing the body structure down the line without having to mess with the values themselves and/or the script.