Append this to /etc/acme-client.conf 

domain example.com {
       alternative names { www.example.com }
       domain key "/etc/ssl/private/example.com.key"
       domain certificate "/etc/ssl/example.com.crt"
       domain full chain certificate "/etc/ssl/example.com.fullchain.pem"
       sign with letsencrypt
}

Replace example.com with your domain and run 

acme-client -v example.com

rcctl reload httpd

Add command to crontab: 

0       0       *       *       *       acme-client example.com

If you use certificates with httpd server: 

0       0       *       *       *       acme-client example.com && rcctl reload httpd