diff --git a/Directory.Build.targets b/Directory.Build.targets
index cc00330b206..eb2e9b5f23d 100644
--- a/Directory.Build.targets
+++ b/Directory.Build.targets
@@ -105,6 +105,7 @@
         BaseOutDir;
         ReactNativeDir;
         ReactNativeWindowsDir;
+        RnwNewArch;
         FollyDir;
         YogaDir;
         WinVer;
diff --git a/change/react-native-windows-14d8afe5-5c05-4205-aeec-11134992e9c2.json b/change/react-native-windows-14d8afe5-5c05-4205-aeec-11134992e9c2.json
new file mode 100644
index 00000000000..c647fcb05d5
--- /dev/null
+++ b/change/react-native-windows-14d8afe5-5c05-4205-aeec-11134992e9c2.json
@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Add multipart/form-data test endpoint",
+  "packageName": "react-native-windows",
+  "email": "julio.rocha@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/vnext/TestWebSite/Microsoft/Office/OfficeJsTests.cs b/vnext/TestWebSite/Microsoft/Office/OfficeJsTests.cs
index 917fc139f9c..6f26f9cafc3 100644
--- a/vnext/TestWebSite/Microsoft/Office/OfficeJsTests.cs
+++ b/vnext/TestWebSite/Microsoft/Office/OfficeJsTests.cs
@@ -1,4 +1,5 @@
 using System.Text;
+using System.Text.RegularExpressions;
 
 namespace Microsoft.Office.Test
 {
@@ -13,5 +14,31 @@ public static async Task Issue4144(HttpContext context)
       var bytes = Encoding.UTF8.GetBytes("Check headers: [Access-Control-Allow-Origin]");
       await response.Body.WriteAsync(bytes);
     }
+
+    public static async Task Issue5869(HttpContext context)
+    {
+      var response = context.Response;
+      response.ContentType = "text/plain";
+      response.StatusCode = 200;
+
+      var request = context.Request;
+
+      string? resBody;
+      if (Regex.IsMatch(request.ContentType!, @"multipart/form-data(;\s+.*)?"))
+      {
+        resBody = $"Multipart form data with {request.Form.Count} entries";
+      }
+      else if (request.ContentType == "application/x-www-form-urlencoded")
+      {
+        resBody = $"URL-encoded form data with {request.Form.Count} entries";
+      }
+      else
+      {
+        resBody = $"Unknown Content-Type [{request.ContentType}]";
+      }
+
+      var bytes = Encoding.UTF8.GetBytes(resBody);
+      await response.Body.WriteAsync(bytes);
+    }
   }
 }
diff --git a/vnext/TestWebSite/Program.cs b/vnext/TestWebSite/Program.cs
index ef80f6b735f..0fe4dd3aca7 100644
--- a/vnext/TestWebSite/Program.cs
+++ b/vnext/TestWebSite/Program.cs
@@ -85,6 +85,10 @@ async Task DefaultRequestDelegate(HttpContext context)
    Microsoft.Office.Test.OfficeJsTests.Issue4144)
   .RequireCors(originPolicyName);
 
+app.MapPost(
+  "/officedev/office-js/issues/5869",
+   Microsoft.Office.Test.OfficeJsTests.Issue5869);
+
 #endregion Request Mappings
 
 await app.RunAsync();