I’m sorry to say, this message API is not suitable for your purpose.
your purpose seems to send message to your customer on behalf of you Kakao business account.
but, Message API provide sending message between users(They must sign up your service and must be friend on KakaoTalk). if you use message API, you can only send message as user account not business account.
so, i recommended biz message which is very suitable. but if you don’t want to use it, there is no way to help you
and i’m not in charge with verifying business app.
if you request partnership for using biz message, this will be resolved, also.