You should consult your Accounting team and/or your book keeper. 

Inventory treatment:

Depending on the country you are in and the accounting regulations covering your business, it could be that the free products need be recorded as current assets at market value to increase the value of your Balance Sheet when they are received. 

• If you are getting extra of a product you already ordered, just receive the additional quantity by increasing the quantity on the Warehouse Receipt for the goods.
• If you are getting a different product, you would add the product and quantities to the Warehouse Receipt (first creating the product with a cost equal to the market value if you haven't received it before).

With both options, the free product(s) will be received into Inventory correctly.

Accounting treatment:

The reason you received them may be important here. If it is a form of rebate it could be considered income. If it is for samples of products they want to encourage you to sell or for products you plan to evaluate for sale or use as raw materials it could be considered a contra expense. If due to their overproduction it could be considered a contra cost of goods. There could also be tax implications for increasing income, reducing expenses or reducing cost of goods before you have sold and/or disposed of the inventory instead as these options have an immediate effect on your Income Statement.

• When creating the Vendor Bill where you have received additional quantities of something you ordered, Odoo will suggest paying for the them, so you will need to offset the value of additional inventory debited by creating a new line to credit to the income / contra expense / contra cost of goods listing by entering the quantity received and a negative unit price. 
• When creating the Vendor Bill where you have received quantities of something not on the Purchase Order, you will need to create both a new additional inventory debit line listing the quantity received with a positive unit price as well as a new line to credit to the income / contra expense / contra cost of goods listing by entering the quantity received and a negative unit price.

Note: if you are still stuck, another solution that might work is to not receive anything into Odoo at all and to just forward the product(s) to your local Odoo office, as we can then feature them (and mention your company) in one of our Odoo eLearning videos, resolving the issue entirely. 😉

If it is a form of rebate, how should treat them ?

For the moment I just add a new line, I put the price and make 100% discount.

Add a Line for the Free Product on Purchase Order:

In the PO, add a new line item for each free product.

Set the Unit Price to Zero:

For the free products, set the unit price to zero. This ensures that the cost of the free products does not affect the total PO amount.

Label Free Products [Optional]:

you can add a note or tag to the line item description to indicate that these are free products.