add private client, mark code_challenge as not required in raml

apis/shopper-login/shopper-login.raml

@@ -1002,7 +1002,7 @@ types:
               The `code_challenge` is created by SHA256 hashing the `code_verifier` and Base64 encoding the resulting hash.
 
               The `code_verifier` should be a high entropy cryptographically random string with a minimum of 43 characters and a maximum of 128 characters.
-            required: true
+            required: false
             type: string
             minLength: 43
             maxLength: 128

src/static/helpers/slasHelper.ts

@@ -379,6 +379,9 @@ export async function loginIDPUser(
     clientId: string;
     siteId: string;
   }>,
+  credentials: {
+    clientSecret?: string;
+  },
   parameters: {
     redirectURI: string;
     hint: string;
@@ -405,6 +408,21 @@ export async function loginIDPUser(
     ...(parameters.dnt !== undefined && {dnt: parameters.dnt.toString()}),
   };
 
+  // Using private client
+  if (credentials.clientSecret) {
+    const authHeaderIdSecret = `Basic ${stringToBase64(
+      `${slasClient.clientConfig.parameters.clientId}:${credentials.clientSecret}`
+    )}`;
+
+    const optionsToken = {
+      headers: {
+        Authorization: authHeaderIdSecret,
+      },
+      body: tokenBody,
+    };
+    return slasClient.getAccessToken(optionsToken);
+  }
+
   return slasClient.getAccessToken({body: tokenBody});
 }