After using Mesh to Metahuman to create a new Metahuman with a custom mesh, Metahuman Creator provides several controls for Region Influences. For the eyes there are two such controls: inner eye and outer eye.
The Inner Eye influence controls just the inside of the eyelids, while the Outer Eye influence controls the outside of the eyelids, the area around the eyes, the lacrimal line and the eyelashes.
This separation of two controls doesn’t seem to work well at the moment, as it doesn’t guarantee the eyelashes will still be placed properly on the eyelids or that the occlusion and lacrimal will be where they should be (also see this recent topic: Metahuman creator: Occlusion and lacrimal not morphed by custom mesh). Even worse, it can cause gaps between the eyeball and the eyelids.
Furthermore, the Outer Eye influence can move and scale the eye itself. After exporting to Unreal, these offsets are not applied to the rig. This means that the eye bone is no longer necessarily at the centre of the eye, causing issues with its rotation. It also causes issues with pupil dilation, where the pupil will move towards or away from the offset of its bone.
I hope these issues can be addressed in a future version!