[BGP] add atribute explanation

Hello all, i want to add an atribute to bgp protocol in the update message. i already added it to the file attrs.c in bgp_attr_table[]. now i want to add this atribute to update message before send it, i read the documentation and i follow the code from the function bgp_encode_attr where it use the function encode of the attribute, but i didn't seen where to configure the data to encode in the attribute. someone could help me understand where i can set this attribute with a default value? i don't need to specify it in the conf file for my project. Thanks everyone who could help me. Mattia