--- a/testing/mozharness/mozharness/mozilla/testing/try_tools.py
+++ b/testing/mozharness/mozharness/mozilla/testing/try_tools.py
@@ -41,16 +41,22 @@ test_flavors = {
 
 class TryToolsMixin(TransferMixin):
     """Utility functions for an interface between try syntax and out test harnesses.
     Requires log and script mixins."""
 
     harness_extra_args = None
     try_test_paths = {}
     known_try_arguments = {
+        '--setenv': {
+            'action': 'append',
+            'dest': 'environment',
+            'default': [],
+            "metavar": "NAME=VALUE",
+        },
         '--tag': {
             'action': 'append',
             'dest': 'tags',
             'default': None,
         },
     }
 
     def _extract_try_message(self):