To authorize HighQ to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used by HighQ is shown below.
HighQ provide support for DKIM. To enable DKIM signing for your custom domain you will have to email HighQ support and enquire for DKIM to be enabled. They will then provide you with the necessary DNS records. Their support address is: email@example.com
Create a free OnDMARC account to test your configuration.