diff --git a/src/guidellm/__main__.py b/src/guidellm/__main__.py
index 85b7cfc0c..1c08a6128 100644
--- a/src/guidellm/__main__.py
+++ b/src/guidellm/__main__.py
@@ -175,7 +175,8 @@ def benchmark():
     default=BenchmarkGenerativeTextArgs.get_default("backend_kwargs"),
     help=(
         "JSON string of arguments to pass to the backend. E.g., "
-        '\'{"api_key": "apikey-*", "verify": false}\''
+        '\'{"api_key": "apikey-*", "verify": false}\' or '
+        '\'{"extras": {"body": {"temperature": 0.6, "top_p": 0.95}}}\' for sampling params.'
     ),
 )
 @click.option(
diff --git a/src/guidellm/backends/openai/http.py b/src/guidellm/backends/openai/http.py
index 9e33f54af..ded568215 100644
--- a/src/guidellm/backends/openai/http.py
+++ b/src/guidellm/backends/openai/http.py
@@ -74,6 +74,13 @@ class OpenAIHttpBackendArgs(BackendArgs):
             )
         },
     )
+    extras: dict[str, Any] | None = Field(
+        default=None,
+        description=(
+            "Optional request arguments to merge into each request (e.g. body fields). "
+            'Use {"body": {"temperature": 0.6, "top_p": 0.95}} to set sampling params.'
+        ),
+    )
 
     @field_validator("request_format")
     @classmethod