Like others are describing, my center cap has a removable disk on the outer face. Behind that is the rubber plug. If your center cap doesn’t have a way to get behind it then, yes, you need to remove the wheel.
If you elect to use the “ez-lube” grease zerk make sure you understand the greasing process. Pump grease SLOWLY and only while the drum is turning. There are several YouTube vids on the procedure.
The Dexter vid posted above is a very good tutorial on repacking.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks