This is managed in the TAR archive given you have the necessary access. 
You need the role: Customs Responsible to perform this.

The countries that can be chosen in the TAR archive and in the pop-up 'TARCREATE' when adding a new code are by default the countries defined in the company's archive (COM), or a country that already has a taric code defined for it.

When creating an EUTARIC/TAR code for a country not listed, one has to load this information into the archive. Once a code is created for a specific country, it will be possible to choose this country when adding more codes to the list. The load used for this is INP/61846 TARICLOAD or by using the INP/14542 ARTUPLOAD.