ActiveCampaign automatically configures SPF for all customers. This means that you don't need to create an SPF record or modify an existing one to work with ActiveCampaign.
Log in to your ActiveCampaign account as your account's admin user.
Click "Settings" located on the left menu.
Click the “Advanced” tab.
Click the “I will manage my own email authentication” option.
Type your sending domain into the DomainKeys Identified Mail (DKIM) field and click the "Generate" button.
ActiveCampaign will generate a TXT Record Name and TXT Record Value for you to enter on your DNS zone.
For more details, see their help docs:
Create a free OnDMARC account to test your configuration