First you need to login to your DNS Exit account to get your API Key (My Account --> My Profile -->API Key)

https://dnsexit.com/Direct.sv?cmd=userApiKey

export JAPI_apikey="ASKMAKM0234m23234xcdfaa"
export JAPI_domain="_acme-challenge.<name of the domain you wish to manage/verify using this script/acme.sh>"

Ok, let's issue a cert now:

acme.sh --issue -d example.com --dns dns_japi

The JAPI_apikey and JAPI_domain will be saved in ~/.acme.sh/account.conf and will be reused when needed.