POST with multipart/form-data using node.js supertest

I was trying to use the Node.js supertest to test some REST API.

request(app)
      .post("/products")
      .set(
        "Authorization",
        "Bearer my jwt token here"
      )
      .set("Content-Type", "multipart/form-data")
      .field("name", "Tomato")
      .field("userId", "5d921d306e96d70a28989127")
      .attach(
        "productImage",
        "D:/NodeJS/node-rest-shop/uploads/1558612339690managing-redis.jpg"
      )
      .expect(201)
      .then(res => {
        const body = res.body;
        expect(body).to.contain.property("message");
        expect(body).to.contain.property("productId");
        expect(body).to.contain.property("date");
        expect(body).to.contain.property("user");
        expect(body).to.contain.property("request");
        done();
      })
      .catch(err => done(err));

.field("userId", userId)

Is there any way to set the value of userId as a variable without setting a string value? It is a MongoDB object id.