Set the USER variable to the account name to backup.

USER="example"
echo "action=backup&append%5Fto%5Fpath=nothing&database%5Fdata%5Faware=yes&email%5Fdata%5Faware=yes&local%5Fpath=%2Fhome%2Fadmin%2Fadmin%5Fbackups&owner=admin&select%30=$USER&type=admin&value=multiple&when=now&where=local" >> /usr/local/directadmin/data/task.queue