If you are a partner, then Marketing Cloud Demo Orgs are part of the partner benefits. so just go ahead and open the Case.
To Obtain Salesforce Marketing Cloud demo org, follow these steps:
Once completed please wait for up to 24 hours to receive log ins.
SAP requires a domain or subdomain, selected by your company, for Salesforce’s exclusive use. This domain should not be in use for existing webpages or internal email. Below are the available options when selecting a domain to represent your brand.
You delegate a domain or subdomain to salesforce nameservers, example: email.mycompany.com. This option requires the domain administrator at your company to create the subdomain and delegate it to salesforce by adding four nameserver (NS) records to the DNS and point them to salesforce nameservers.
Please see the following help page for more details on domain delegation.
Custom Domain or Subdomain Delegation in Marketing Cloud
You will receive email to Access Your Marketing Cloud Account provided with Username & Temporary Password.
Login with https://mc.exacttarget.com & do setup of demo org tailored to your organization’s need.