Got you. Sorry, misunderstood the question. LRServer is something new to me, but can see the JSON that you're refering to in this sample:
https://roadsandhighwayssample.esri.com/server/rest/services/RoadsHighways/RelocateEvents/MapServer/...
I would assume that it's something specific to the Linear Referencing workflow? This is an area I haven't worked in and have never noted a requirement under a normal FeatureLayer workflow.
I'll stand away and let someone else (more experienced in that area) respond.
Good luck on sorting it, sorry I can't help.
Scott Tansley
https://www.linkedin.com/in/scotttansley/