diff --git a/src/openforms/registrations/contrib/json/config.py b/src/openforms/registrations/contrib/json/config.py
index 091cc529e0..7b7c9580eb 100644
--- a/src/openforms/registrations/contrib/json/config.py
+++ b/src/openforms/registrations/contrib/json/config.py
@@ -15,20 +15,22 @@ class JSONOptionsSerializer(JsonSchemaSerializerMixin, serializers.Serializer):
         queryset=Service.objects.all(),
         label=_("Service"),
         help_text=_("Which service to use."),
+        required=True,
     )
-    # TODO-4098: show the complete API endpoint as a (tooltip) hint after user entry? Might be a front-end thing...
+    # TODO-4098: show the complete API endpoint as a (tooltip) hint after user entry?
+    #  Might be a front-end thing...
     relative_api_endpoint = serializers.CharField(
         max_length=255,
         label=_("Relative API endpoint"),
         help_text=_("The API endpoint to send the data to (relative to the service API root)."),
         allow_blank=True,
+        required=False,
     )
     form_variables = serializers.ListField(
         child=FormioVariableKeyField(max_length=50),
         label=_("Form variable key list"),
-        help_text=_(
-            "A comma-separated list of form variables (can also include static variables) to use."
-        )
+        help_text=_("A list of form variables (can also include static variables) to use."),
+        required=True,
     )
 
 
diff --git a/src/openforms/registrations/contrib/json/plugin.py b/src/openforms/registrations/contrib/json/plugin.py
index 13f4a9c843..c3276a7c1c 100644
--- a/src/openforms/registrations/contrib/json/plugin.py
+++ b/src/openforms/registrations/contrib/json/plugin.py
@@ -20,6 +20,8 @@ class JSONRegistration(BasePlugin):
 
     def register_submission(self, submission: Submission, options: JSONOptions) -> None:
         # TODO-4908: the email plugin works with a EmailConfig singleton model. Is that useful here?
+        #  Doesn't look like it. Seems to be used as a model for default configuration options, which
+        #  which wouldn't really be useful for this plugin
 
         # TODO-4908: any other form field types that need 'special attention'?
 
@@ -40,8 +42,9 @@ def register_submission(self, submission: Submission, options: JSONOptions) -> N
             variable.key: variable.initial_value for variable in static_variables
         }
 
-        # TODO-4908: what should the behaviour be when a form
-        #  variable is not in the data or static variables?
+        # TODO-4908: what should the behaviour be when a form variable is not in the data or static variables?
+        #  Raising an error probably a good idea, the form variable is currently just set to None in the
+        #  resulting values dict
         # Update values dict with relevant form data
         values.update({
             form_variable: submission.data.get(
@@ -58,7 +61,7 @@ def register_submission(self, submission: Submission, options: JSONOptions) -> N
         # TODO-4098: is the service type relevant here?
         with build_client(service) as client:
             response = client.post(
-                options["relative_api_endpoint"],
+                options.get("relative_api_endpoint", ""),
                 json=json,
                 headers={"Content-Type": "application/json"},
             )