lnrpc: set either payHash or setId depending on payreq type

lnrpc/routerrpc/router_backend.go

@@ -702,10 +702,32 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 			payIntent.MaxParts = 1
 		}
 
+		if payReq.Features.HasFeature(lnwire.AMPOptional) {
+			// Generate random SetID and root share.
+			var setID [32]byte
+			_, err = rand.Read(setID[:])
+			if err != nil {
+				return nil, err
+			}
+
+			var rootShare [32]byte
+			_, err = rand.Read(rootShare[:])
+			if err != nil {
+				return nil, err
+			}
+			err := payIntent.SetAMP(&routing.AMPOptions{
+				SetID:     setID,
+				RootShare: rootShare,
+			})
+			if err != nil {
+				return nil, err
+			}
+		} else {
 			err = payIntent.SetPaymentHash(*payReq.PaymentHash)
 			if err != nil {
 				return nil, err
 			}
+		}
 
 		destKey := payReq.Destination.SerializeCompressed()
 		copy(payIntent.Target[:], destKey)