Go to Marketing > Promotions > Cart Price Rules, then click on Add New Rule.

Setup the Rule Information and the Conditions according to your needs.

Scroll down to the tab Actions and select an action in the FedEx field:

Save the modifications.

Go to Stores > Settings > Configurations > Sales > Shipping Methods > FedEx

In the "Free Method" option, select the method that will be displayed as Free when the Cart Rule has its conditions matched:

Attention: Make sure that the selected Free Method is also enabled in the Allowed Methods list, otherwise the free shipping option won't be displayed.