At the moment, we're not offering Dedicated IP Addresses through Growably.
So, if you're wanting to send your emails through a Dedicated IP, you'll need to add your own External SMTP Provider (e.g. Mailgun, SendGrid, AmazonSES etc).
You can find more details here.
Keep in mind that a dedicated IP for SMTP is usually only recommended if you're sending more than 100,000+ emails every month.
Until you reach that point, you're normally better off leveraging the reputation of the Shared IP Addresses of whatever email sending provider you're using (Growably, ActiveCampaign, Keap, Hubsoft etc).