feat(authorize): memoTag field on AuthorizeBody

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ## [Unreleased]
 
+### Added
+
+- `AuthorizeBody` gains `MemoTag` (`memoTag`) — a Notabene extension (TAIP-4
+  defines no memo-tag carriage) for the settlement memo/destination tag, as a
+  fallback to embedding it as a `:tag` suffix on `settlementAddress`.
+
 ## [0.6.0] - 2026-07-08
 
 ### Added

authorize.go

@@ -14,6 +14,7 @@ type AuthorizeBody struct {
 	Type              string   `json:"@type"`
 	SettlementAddress string   `json:"settlementAddress,omitempty"`
 	SettlementAsset   string   `json:"settlementAsset,omitempty"`
+	MemoTag           string   `json:"memoTag,omitempty"`
 	Amount            string   `json:"amount,omitempty"`
 	Expiry            string   `json:"expiry,omitempty"`
 	ApprovedTypes     []string `json:"approvedTypes,omitempty"`

authorize_test.go

@@ -43,7 +43,8 @@ func TestAuthorizeBody_JSONRoundTrip(t *testing.T) {
 	body := AuthorizeBody{
 		Context:           TAPContext,
 		Type:              TypeAuthorize,
-		SettlementAddress: "eip155:1:0x1234",
+		SettlementAddress: "xrpl:0:rN7n3473SaZBCG4dFL83w7a1RXtXtbk2D8",
+		MemoTag:           "998877",
 		Amount:            "100.00",
 	}
 
@@ -59,6 +60,23 @@ func TestAuthorizeBody_JSONRoundTrip(t *testing.T) {
 	if got.SettlementAddress != body.SettlementAddress {
 		t.Errorf("mismatch: %+v", got)
 	}
+	if got.MemoTag != body.MemoTag {
+		t.Errorf("MemoTag mismatch: %+v", got)
+	}
+}
+
+func TestAuthorizeBody_MemoTagOmittedWhenEmpty(t *testing.T) {
+	data, err := json.Marshal(AuthorizeBody{Context: TAPContext, Type: TypeAuthorize})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	var raw map[string]json.RawMessage
+	if err := json.Unmarshal(data, &raw); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if _, ok := raw["memoTag"]; ok {
+		t.Errorf("memoTag should be omitted when empty: %s", data)
+	}
 }
 
 func TestAuthorize_TestVectorValid(t *testing.T) {