Assuming that curl is in /usr/bin on your system, you could add the following command:
/usr/bin/curl -sk https://mydomain.com/?__wallets_action=do_cron&__wallets_apiversion=3&__wallets_cron_nonce=0227e9c5029dxxxxxxxxxxxxx > /dev null 2>&1
The full crontab entry for running this every five minutes would be:
*/5 * * * * /usr/bin/curl -sk https://mydomain.com/?__wallets_action=do_cron&__wallets_apiversion=3&__wallets_cron_nonce=0227e9c5029dxxxxxxxxxxxxx > /dev null 2>&1