Dynamics 365 - Add System Customizer role

This entry assumes that you have or at least you created a trial for Dynamics 365 CRM and signed in.
You can create a trial here: https://port.crm.dynamics.com/G/TrialInstances/TrialInstancePicker.aspx

1. Click the Gear located in the top right corner
2. Click Advanced Settings
3. Click Settings and then Security
4. Click Users
5. Select the desired user and click Manage Roles
6. Select System Customizer