Add the following dependency

.package(url: "https://github.com/apple/app-store-server-library-swift.git", .upToNextMinor(from: "0.1.0")),

import AppStoreServerLibrary

let issuerId = "99b16628-15e4-4668-972b-eeff55eeff55"
let keyId = "ABCDEFGHIJ"
let bundleId = "com.example"
let encodedKey = try! String(contentsOfFile: "/path/to/key/SubscriptionKey_ABCDEFGHIJ.p8")
let environment = Environment.sandbox

// try! used for example purposes only
let client = try! AppStoreServerAPIClient(signingKey: encodedKey, keyId: keyId, issuerId: issuerId, bundleId: bundleId, environment: environment)

let response = await client.requestTestNotification()
switch response {
case .success(let response):
    print(response.testNotificationToken)
case .failure(let errorCode, let apiError, let causedBy):
    print(errorCode)
    print(apiError)
    print(causedBy)
}

import AppStoreServerLibrary

let bundleId = "com.example"
let appleRootCAs = loadRootCAs() // Specific implementation may vary
let enableOnlineChecks = true
let environment = Environment.sandbox

// try! used for example purposes only
let verifier = try! SignedDataVerifier(rootCertificates: appleRootCAs, bundleId: bundleId, appAppleId: nil, environment: environment, enableOnlineChecks: enableOnlineChecks)

let notificationPayload = "ey..."
let notificationResult = await verifier.verifyAndDecodeNotification(signedPayload: notificationPayload)
switch notificationResult {
case .valid(let decodedNotificaiton):
    ...
case .invalid(let error):
    ...
}

import AppStoreServerLibrary

let issuerId = "99b16628-15e4-4668-972b-eeff55eeff55"
let keyId = "ABCDEFGHIJ"
let bundleId = "com.example"
let encodedKey = try! String(contentsOfFile: "/path/to/key/SubscriptionKey_ABCDEFGHIJ.p8")
let environment = Environment.SANDBOX

// try! used for example purposes only
let client = try! AppStoreServerAPIClient(signingKey: encodedKey, keyId: keyId, issuerId: issuerId, bundleId: bundleId, environment: environment)

let appReceipt = "MI..."
let receiptUtil = ReceiptUtility()
let transactionIdOptional = receiptUtil.extractTransactionId(appReceipt: appReceipt)
if let transactionId = transactionIdOptional {
    var transactionHistoryRequest = TransactionHistoryRequest()
    transactionHistoryRequest.sort = TransactionHistoryRequest.Order.ascending
    transactionHistoryRequest.revoked = false
    transactionHistoryRequest.productTypes = [TransactionHistoryRequest.ProductType.autoRenewable]

    var response: HistoryResponse?
    var transactions: [String] = []
    repeat {
        let revisionToken = response?.revision
        let apiResponse = await client.getTransactionHistory(originalTransactionId: transactionId, revision: revisionToken, transactionHistoryRequest: transactionHistoryRequest)
        switch apiResponse {
        case .success(let successfulResponse):
            response = successfulResponse
        case .failure:
            // Handle Failure
            throw
        }
        if let signedTransactions = response?.signedTransactions {
            transactions.append(contentsOf: signedTransactions)
        }
    } while (response?.hasMore ?? false)
    print(transactions)
}

import AppStoreServerLibrary

let keyId = "ABCDEFGHIJ"
let bundleId = "com.example"
let encodedKey = try! String(contentsOfFile: "/path/to/key/SubscriptionKey_ABCDEFGHIJ.p8")

let productId = "<product_id>"
let subscriptionOfferId = "<subscription_offer_id>"
let applicationUsername = "<application_username>"

// try! used for example purposes only
let signatureCreator = try! PromotionalOfferSignatureCreator(privateKey: encodedKey, keyId: keyId, bundleId: bundleId)

let nonce = UUID()
let timestamp = Int64(Date().timeIntervalSince1970) * 1000
let signature = signatureCreator.createSignature(productIdentifier: productIdentifier, subscriptionOfferID: subscriptionOfferID, applicationUsername: applicationUsername, nonce: nonce, timestamp: timestamp)
print(signature)