Create Foreign Key without implementing Primary Key in SQL Server

i want to create two table

CREATE TABLE [dbo].[User](
	[UserID] [int] IDENTITY(1,1) NOT NULL,
	[UserName] [varchar](30) NOT NULL,
	[CreatedByID] [int] NULL
)

CREATE TABLE [dbo].[Product](
	[ProductID] [int] IDENTITY(1,1) NOT NULL,
	[ProductName] [varchar](30) NOT NULL,
	[CreatedByID] [int] NULL
)

In product table how to set foreign key without a primary key.becouse i want to save createdByID same record EXample: UserID:1, UserName:admin, CreatedByID:1 UserID:2, UserName:client, CreatedByID:1