No — the GTIP / HS code is not required, and you can publish your demand with it left blank.
The field sits behind the Enter export info button under each product row on the Products step. The Export information dialog holds GTIP / HS Code, Origin and Brand, and each label is marked (optional). You can close the dialog without filling anything in, or skip opening it altogether — the row simply shows "No export info entered". The fields a product line genuinely requires are product name, quantity and unit.
It is still worth adding. The HS code identifies the customs tariff position, so suppliers can price duties and freight properly. That makes the offers you receive more realistic, easier to compare side by side, and less likely to surprise you with extra cost after you accept one.
There is no format restriction, so a partial code is fine (for example 6802.91). Whatever you enter appears to suppliers viewing your demand in the HS code column of the product table. If no product carries one, that column is not shown at all.
Also asked as: Can I leave the HS code empty? · Is the HS code mandatory? · What if I don't know my GTIP code? · Is the export information section required?