Humm not sure what you are attempting. The need for IK should be built into the control rig design and not make use of the joints marked as IK as it is a misleading as to their purpose as being included as part of the hierarchy.
There are really only two practical reason for Epic to include them.
- Legacy as allowing past animation sets to work with the current UE5 design as to exaptation.
- To act as helper or accessory joints when authoring animations when using external animation applications such as Motionbuilder.
Typically when added as a helper the IK joint is “position constrained” to the appropriate joint like IK_gun would be constrained to the right hand but would be parented to the hand root which in turn parented to the character root. Once plotted the animation is applied to the IK joint so as to allow object interaction like switching weapon hands for things like operating a bolt action rifles.
Once imported into UE5 there really is no other use as part of the control rig design as UE5 does not respect the constraint system of any secondary animation application so to make it work as part of the control rig you would have to make your own position constraint