1) Create a text file C:\OpenLDAP\sysconf\ldap.conf:
TLS_REQCERT never
TLS_CACERT C:\OpenLdap\sysconf\certs\myldapcert.pem
2) Extract certificate from LDAP in PEM format and save as myldapcert.pem
openssl s_client -connect ldap.mydomain.com:636 -showcerts
3) Copy the certificate (myldapcert.pem) to C:\OpenLdap\sysconf\certs\myldapcert.pem