Add support for sending MSISDN tokens to alternate URLs

Part of https://github.com/vector-im/riot-web/issues/10923

src/base-apis.js

@@ -1963,7 +1963,7 @@ MatrixBaseApis.prototype.requestMsisdnToken = async function(
 };
 
 /**
- * Submits an MSISDN token to the identity server
+ * Submits a MSISDN token to the identity server
  *
  * This is used when submitting the code sent by SMS to a phone number.
  * The ID server has an equivalent API for email but the js-sdk does
@@ -2013,6 +2013,41 @@ MatrixBaseApis.prototype.submitMsisdnToken = async function(
     }
 };
 
+/**
+ * Submits a MSISDN token to an arbitrary URL.
+ *
+ * This is used when submitting the code sent by SMS to a phone number in the
+ * newer 3PID flow where the homeserver validates 3PID ownership (as part of
+ * `requestAdd3pidMsisdnToken`). The homeserver response may include a
+ * `submit_url` to specify where the token should be sent, and this helper can
+ * be used to pass the token to this URL.
+ *
+ * @param {string} url The URL to submit the token to
+ * @param {string} sid The sid given in the response to requestToken
+ * @param {string} clientSecret A secret binary string generated by the client.
+ *                 This must be the same value submitted in the requestToken call.
+ * @param {string} msisdnToken The MSISDN token, as enetered by the user.
+ *
+ * @return {module:client.Promise} Resolves: Object, currently with no parameters.
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ */
+MatrixBaseApis.prototype.submitMsisdnTokenOtherUrl = function(
+    url,
+    sid,
+    clientSecret,
+    msisdnToken,
+) {
+    const params = {
+        sid: sid,
+        client_secret: clientSecret,
+        token: msisdnToken,
+    };
+
+    return this._http.requestOtherUrl(
+        undefined, "POST", url, undefined, params,
+    );
+};
+
 /**
  * Gets the V2 hashing information from the identity server. Primarily useful for
  * lookups.