You need a small tapered tip on your grease gun. Usually there's a semi spherical depression with a little ball check in the middle. The grease gun tips come with various radii on the nose, and it should be close to a match. If not, a bigger radius on the tip than the depression will work. Be sure to wipe out the fitting so you don't blow grit into the bearing when pumping grease.
You need to push it into the depression the whole time you are pumping, since it's not self locking like a Zerk fitting. Don't pump too much such that it blows out the seals.
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