how to specify default value for foreign key reference in PostgreSQL

Suppose there is a two tables "User" and "Product", so there is also a foreign key, that means, that one user can have multiple products, like:

CREATE TABLE "user" (

  id BIGSERIAL NOT NULL,
  Name VARCHAR(100) NOT NULL,

  ProductId PRIMARY KEY,  
  Products FOREIGN KEY(ProductId) REFERENCES product(id)

)

CREATE TABLE "product"(
    id BIGSERIAL NOT NULL PRIMARY KEY,
    product_name VARCHAR(100) NOT NULL,
)

The problem is when I'm creating a new user I don't specify ProductId, (because for example product does not exist yet) it always it throws an exception, that ProductId has a unique constraint and should be not null.

So What I Need to Put Inside ProductId?

Because I don't specify any value for it when creating User, it throws unique constraint error, but without this Field I cannot have a foreign key between two tables, so it cannot be removed.

So, the question is "How can I Specify Default Value for This Reference "ProductId" Field in My "User" model, when creating an user object, in order to avoid this unique constraint error."