diff --git a/README.md b/README.md
index 7d8c8be..7ba041a 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,27 @@ Once installed and the server is restarted, there is no configuration needed. Si
 
 ---
 
+### Developer API: Registering Custom Fields
+
+If you have built your own Redmine plugins that add custom columns to the issues database table, you can register those fields to be automatically copied when a subtask is created.
+In your custom plugin's init.rb, append your field names (as symbols) to the SubtaskFieldCopier.registered_fields array.
+It is best practice to wrap this in a Rails.configuration.to_prepare block and check if the module is defined, ensuring your plugin won't crash if the Subtask Field Copier is uninstalled:
+
+```Ruby
+Rails.configuration.to_prepare do
+  if defined?(SubtaskFieldCopier)
+    # Register your custom issue fields here
+    SubtaskFieldCopier.registered_fields << :vehicle
+    SubtaskFieldCopier.registered_fields << :customer
+    
+    # Ensure there are no duplicates (useful for development hot-reloading)
+    SubtaskFieldCopier.registered_fields.uniq!
+  end
+end
+```
+
+---
+
 ## License
 
 > The MIT License (MIT)
diff --git a/lib/subtask_field_copier.rb b/lib/subtask_field_copier.rb
index 7d621a9..a433ce1 100644
--- a/lib/subtask_field_copier.rb
+++ b/lib/subtask_field_copier.rb
@@ -10,6 +10,9 @@
 
 module SubtaskFieldCopier
 
+  mattr_accessor :registered_fields
+  self.registered_fields = []
+
   def self.setup
    unless IssuesController.ancestors.include?(SubtaskFieldCopier::Patches::IssuesControllerPatch)
       IssuesController.prepend SubtaskFieldCopier::Patches::IssuesControllerPatch
diff --git a/lib/subtask_field_copier/patches/issues_controller_patch.rb b/lib/subtask_field_copier/patches/issues_controller_patch.rb
index d8505e5..58e04b8 100644
--- a/lib/subtask_field_copier/patches/issues_controller_patch.rb
+++ b/lib/subtask_field_copier/patches/issues_controller_patch.rb
@@ -28,17 +28,15 @@ module SubtaskFieldCopier
             end
             @issue.custom_field_values = cf_values
 
-            # --- COPY CUSTOMER, VEHICLE & ESTIMATE---
-            if @issue.respond_to?(:customer=) && parent_issue.respond_to?(:customer)
-              @issue.customer = parent_issue.customer 
-            end
-            
-            if @issue.respond_to?(:vehicle=) && parent_issue.respond_to?(:vehicle)
-              @issue.vehicle = parent_issue.vehicle 
-            end
-
-            if @issue.respond_to?(:estimate=) && parent_issue.respond_to?(:estimate)
-              @issue.estimate = parent_issue.estimate 
+            # --- COPY REGISTERED FIELDS ---
+            # Iterate over any fields registered by other plugins
+            SubtaskFieldCopier.registered_fields.each do |field|
+              setter = "#{field}="
+              
+              # Use metaprogramming to check if the setter/getter exists, then apply
+              if @issue.respond_to?(setter) && parent_issue.respond_to?(field)
+                @issue.send(setter, parent_issue.send(field))
+              end
             end
           end
         end