The Legend graphics URL of WMS layer can be found at the setting of WMS layer at GeoserverÂ
For example, here is a layer setting at layers.js in GLTG
"properties": { "title": "River Reaches", "id": "gltg:gltg-pools", "wms": "https://gltg.ncsa.illinois.edu/geoserver/wms", "opacity": 0.9, "checked": false, "showLayerLegend": false, "layerLegendTitle": "", "layerLegendContents": "" }
coming