To authorize Bullhorn to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanisms used by Bullhorn are shown below.
Bullhorn requires that the above SPF changes be enabled before DKIM.
Once that is completed, create the following CNAME record with your DNS provider.
Then contact Bullhorn support and let them know that this record is created, and request to enable DKIM for your account.
Create a free OnDMARC account to test your configuration.