VRChat U# example for setting a singleton manager reference on many objects, using a button.

PropManagerEditor.cs

using UdonSharpEditor;
using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(PropManager)), CanEditMultipleObjects]
public class PropManagerEditor : UnityEditor.Editor
{
    public override void OnInspectorGUI()
    {
        if (UdonSharpGUI.DrawDefaultUdonSharpBehaviourHeader(target)) return;
        
        var propManager = (PropManager)target;

        if (GUILayout.Button("Add Manager to Props"))
        {
            var props = FindObjectsOfType<Prop>(true);
            foreach (var propComponent in props)
            {
                var prop = new SerializedObject(propComponent);
                prop.FindProperty("propManager").objectReferenceValue = propManager;
                prop.ApplyModifiedProperties();
            }
        }
        GUILayout.Space();

        // DrawDefaultInspector adds all the other fields. So you don't need to customize
        // everything in the inspector!
        DrawDefaultInspector();
    }
}