Stop in for a cup of coffee

The one I pictured is a little messy but it get's the job done. Screw a zerk into the end and you can snap it onto your grease gun coupler.
Never used a slide on type like that. The regular 90s work OK. But if it’s your own equipment, I would change th fitting to a 45 or 90. The it’s greasable with a straight tip.