Custom domains and DKIM sender domain verification are both domain related settings but they are not related: Custom domains are for branding, DKIM sender domain verification is for inbox delivery. However, since setup for both needs to be completed by your IT team it's recommended they are configured at the same time.
What is DKIM and why it's needed?
If you're unfamiliar with DKIM, read this article first: What's DKIM and why it's needed
Setup of DKIM sender domain verification
DKIM setup is a one-time task involving multiple steps involving your IT team.
Step 1: Choose the domain you'll be sending emails from. This is the part after the @ sign in email addresses. This is the sending domain. For example if you send emails from email@example.com then the sending domain is mywebsite.com
Step 2: Request the "DKIM setup code" for your sending domain from Envoke support. Just say: "Please send DKIM details for the following domain: sample.com"
Step 3: Envoke will send you the DKIM code within two business days at most.
Step 4: Send a request to your IT team with the following instructions:
We're setting up an account with Envoke.com to send bulk emails. In order to authenticate our sender domain DKIM needs to be implemented.
Please add a TXT record to our domain [enter domain name here] using the following details
[Paste the code you received from Envoke in Step 3.]
You can verify setup at https://dkimcore.org/c/keycheck using selector = nvk
Step 5: Your IT team completes the setup.
Step 6: Let Envoke support know and we'll confirm the setup is done correctly.