Active Campaign manages SPF on their own and therefore there is no need to add them to your SPF record. However, they may ask to add them to your SPF record for validaiton purposes. Here is the SPF mechanism used by Active Campaign.
Active Campaign support DKIM signing for your custom domain in order to authenticate your emails and pass DMARC validation.
For more information on the above and how to configure DKIM please click on the button below.
Create a free OnDMARC account to test your configuration.