How to add something to the body of request inside an Angular interceptor?

Here I'm able to modify the header as there are multiple tutorials present regarding this feature but:

@Injectable()
export class MyFirstInterceptor implements HttpInterceptor {

    constructor(private currentUserService: CurrentUserService) { }

    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        console.log(JSON.stringify(req));

        const token: string = this.currentUserService.token;

        if (token) {
            req = req.clone({ headers: req.headers.set('Authorization', 'Bearer ' + token) });
        }

        if (!req.headers.has('Content-Type')) {
            req = req.clone({ headers: req.headers.set('Content-Type', 'application/json') });
        }

        req = req.clone({ headers: req.headers.set('Accept', 'application/json') });
        return next.handle(req);
    }
}

But in my case there's a token which I need to add the request body instead of the request header so is there any method to modify the body.

from How to add something to the body of request inside an Angular interceptor?