--- a/dom/base/ChromeUtils.cpp
+++ b/dom/base/ChromeUtils.cpp
@@ -776,16 +776,36 @@ void ChromeUtils::GetBaseDomainFromParti
                                                     pkBaseDomain, port)) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
 
   aBaseDomain = pkBaseDomain;
 }
 
+/* static */
+void ChromeUtils::GetPartitionKeyFromURL(dom::GlobalObject& aGlobal,
+                                         const nsAString& aURL,
+                                         nsAString& aPartitionKey,
+                                         ErrorResult& aRv) {
+  nsCOMPtr<nsIURI> uri;
+  nsresult rv = NS_NewURI(getter_AddRefs(uri), aURL);
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aPartitionKey.Truncate();
+    aRv.Throw(rv);
+    return;
+  }
+
+  mozilla::OriginAttributes attrs;
+  attrs.SetPartitionKey(uri);
+
+  aPartitionKey = attrs.mPartitionKey;
+}
+
 #ifdef NIGHTLY_BUILD
 /* static */
 void ChromeUtils::GetRecentJSDevError(GlobalObject& aGlobal,
                                       JS::MutableHandleValue aRetval,
                                       ErrorResult& aRv) {
   aRetval.setUndefined();
   auto runtime = CycleCollectedJSRuntime::Get();
   MOZ_ASSERT(runtime);

--- a/dom/base/ChromeUtils.h
+++ b/dom/base/ChromeUtils.h
@@ -124,16 +124,21 @@ class ChromeUtils {
            aA.mPrivateBrowsingId == aB.mPrivateBrowsingId;
   }
 
   static void GetBaseDomainFromPartitionKey(dom::GlobalObject& aGlobal,
                                             const nsAString& aPartitionKey,
                                             nsAString& aBaseDomain,
                                             ErrorResult& aRv);
 
+  static void GetPartitionKeyFromURL(dom::GlobalObject& aGlobal,
+                                     const nsAString& aURL,
+                                     nsAString& aPartitionKey,
+                                     ErrorResult& aRv);
+
   // Implemented in js/xpconnect/loader/ChromeScriptLoader.cpp
   static already_AddRefed<Promise> CompileScript(
       GlobalObject& aGlobal, const nsAString& aUrl,
       const dom::CompileScriptOptionsDictionary& aOptions, ErrorResult& aRv);
 
   static MozQueryInterface* GenerateQI(const GlobalObject& global,
                                        const Sequence<JS::Value>& interfaces,
                                        ErrorResult& aRv);

--- a/dom/chrome-webidl/ChromeUtils.webidl
+++ b/dom/chrome-webidl/ChromeUtils.webidl
@@ -312,16 +312,28 @@ partial namespace ChromeUtils {
    * Returns the empty string for an empty partitionKey.
    * Throws for invalid partition keys.
    */
   [Throws]
   DOMString
   getBaseDomainFromPartitionKey(DOMString partitionKey);
 
   /**
+   * Returns the partitionKey for a given URL.
+   *
+   * The function will treat the URL as a first party and construct the
+   * partitionKey according to the scheme, site and port in the URL.
+   *
+   * Throws for invalid urls.
+   */
+  [Throws]
+  DOMString
+  getPartitionKeyFromURL(DOMString url);
+
+  /**
    * Loads and compiles the script at the given URL and returns an object
    * which may be used to execute it repeatedly, in different globals, without
    * re-parsing.
    */
   [NewObject]
   Promise<PrecompiledScript>
   compileScript(DOMString url, optional CompileScriptOptionsDictionary options = {});