Not trying to sound cheeky but maybe at eQSL.cc they have an FAQ how their system works? I used eQSL many years ago but I don’t recall them having a message field. Dave W7UUU
I believe you can as part of your ADIF upload using the "COMMENT" field as specified by ADIF.ORG. Assuming eQSL supports it, then this is where you should write your "Tks for QSO! 73 de DW" or whatever you want to send.