The below will need to be added in Incognito Mode, if not done in this mode then you will not be able to complete the setup.
Note - You cannot make any changes to this once saved as if you go back into the details of the created account it’s possible the account could bug and no longer work, so you need to make sure its set to Active and Live
Configuration → Couriers
FedexRest (there are 3 Fedex options, if you hover over each you can which is the FedexRest, it is usually the third in the list)→ Add Account
Account number → your FEDEX shipping account number.
Enter the address details in street lines, city, pin code, state or province code section. The Test side can be the same as the Live side. The Country Code and State should be GB.
Scroll to bottom of FEDEX Eula to enable accept checkbox, then accept EULA agreement, and click on validate Address.
One box will be displayed at bottom of pop-up to enter PIN for MFA validation, enter PIN received on selected validation method and hit submit pin. You need to enter this PIN in the second you get it or it may timeout.
This on successful registration should add an account to use for FEDEX shipping.