There is a 3 step process that you need to follow and this process relies upon you having enough permissions in your organisations M365 Portal and Power Platform Portal to achieve these tasks.
Step A - Acquire licenses for the trial
1. Open the M365 Admin Portal > Billing > Purchase Services (https://admin.microsoft.com/AdminPortal/Home#/homepage)
3. In 'View by Category' unselect 'Recommended' - this removes a filter from the list of apps that are shown
4. Now click on the filter to show Dynamics 365 - this applies a filter to just show you D365 apps :)
5. Select the Dynamics 365 Supply Chain Management Trial
6. Press Details then press Start free trial.
Step B - Assign the Licenses to your users
7. Open the M365 Admin Portal>Users>Active Users
8. Assign the new license to the user account that will be used to provision the new environment and anyone else that will use it.
Step C - Create a Trial environment and deploy D365SCM
9. Open Power Platform Admin Portal > Environments > New Environment (https://admin.powerplatform.microsoft.com/environments)
10. Enter a Name, Select Region 'United States' - it has to be this at the moment as its the only region these trials are enabled for at the time of writing.
11. Select the Type as 'Trial (Subscription-based)'
12. Press Next
13. Add a security group (mandatory)
13. Enable D365 Apps sets to Yes
14. In the box 'Automatically deploy these apps' choose choose 'Supply Chain Management (Preview)
The above steps are shown as below - sorry for the big image (running out of time before family event :)
15. Press Save - now go and have a long walk (it took 17 mins for Microsoft to do this step this time - sometimes it might be longer) or a cup of tea whilst your new environment is provisioned.
When complete you will see the URL for your new F&O environment as below.....
Note: Remember that Microsoft state:
"The expiration of subscription-based trial (also know as an admin trial) environments is tied to the expiration of the subscription. Therefore, subscription-based trial environments cannot be independently extended. There are limited options for extending admin trial subscriptions which affects extending the expiration of any associated trial environments.
To extend a subscription-based (admin) trial:
Admins can request a single extension in the Microsoft 365 admin center. See Try or buy a Microsoft 365 for business subscription.
There is only one self-service extension allowed per offer-based trial environment. To further retain the environment, it must be converted to production.