Calculation of EAN13:

You can choose first 12 numbers. The 13th is computed. Let x be the sum of numbers on odd positions Let y be the sum of numbers on even positions Let z be x+(y*3) Let z' be z rounded up (73 to 80 for example) Then 13th number is z' - z

I show you with 593253012713 example.I selected a 12 digit number and calculating the thirteen digit. So (5+3+5+0+2+1) = 16, (9+2+3+1+7)*3 = 75. Therefore we have 16 + 75 = 91. Then rounded(91) - 91 is 100 - 91 = 9. So the last digit should be 9 and then it's a valid EAN code 5932530127139

OR

Install point_of_sale module and this will enable Button 'Set a Custom EAN' below this EAN13 field and with help of this button you can add your custom EAN13 no. But this is not recommended.