Making a callout To Marketing cloud from Service Cloud External Services

Regarding how to authenticate with SFMC, here is what you need:

• Within SFMC: Go into Administration > Installed Packages and find/create the installed package (server-to-server type) you are using to authenticate and make REST calls. More info here.
• Next step is to define/check the API Integration Permission Scopes for your installed package. More info here.

To use the transactional API you will need below scopes defined:

-- Email - Read, Write, Send

-- Lists and Subscribers - Read, Write

-- Data Extensions - Read, Write

-- Interactions - Read, Write, Execute

• Once you've setup the package you'll be able to retrieve ClientId, ClientSecret and Authentication URI. More info here.

• Below is a sample payload from the official SF documentation:

Host: https://YOUR_SUBDOMAIN.auth.marketingcloudapis.com
POST /v2/token
Content-Type: application/json

{
"grant_type": "client_credentials",
"client_id": "7a9j47upktedde30uedl822p",
"client_secret": "1955278925675241571",
"scope": "email_read email_write email_send",
"account_id": "12345"
}

• With regards to flow setup I am not sure what is involved but if you could manage to setup transactional API requests then this one shouldn't be vastly different.