In order to add a primary and foreign keys to a table when creating a table, we do the following:

CREATE TABLE products (
    product_id UUID GENERATED ALWAYS AS IDENTITY,
    product_name VARCHAR(500),
    sku VARHCAR(50),
    company_id UUID,
    quantity INT,
    PRIMARY KEY(product_id),
    CONSTRAINT fk_company_id
      FOREIGN KEY(company_id)
        REFERENCES companies(company_id)
);