'API Key Missing' when making Mailchimp API call from Swift 5 iOS App - ios

Question

I am trying to add a subscriber to my mailing list from my Swift 5 iOS app. I am seeing the following error when trying to do this:
{
detail = "Your request did not include an API key.";
instance = "3f4cb654-c674-4a97-adb8-b4eb6d86053a";
status = 401;
title = "API Key Missing";
type = "http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/";
}
Of course this indicates that I am missing my API Key, however I am specifying it in the Authorization header (see below code). I have tried a mix of the answer here and the guide here but I'm not having much luck so far. Here's my current code for setting up the request:
let mailchimpAPIURL = "https://us3.api.mailchimp.com/3.0"
let requestURL = NSURL(string: mailchimpAPIURL)!
let apiCredentials = "anystring:<API_KEY>"
let loginData = apiCredentials.data(using: String.Encoding.utf8)!.base64EncodedString()
let params = [
"list_id": "<LIST_ID>",
"email_address": email,
"status": "subscribed",
"merge_vars": [
"FNAME": firstName,
"LNAME": lastName
]
] as [String: Any]
let request = NSMutableURLRequest(url: requestURL as URL)
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
request.setValue("Basic \(loginData)", forHTTPHeaderField: "Authorization")
do {
request.httpBody = try JSONSerialization.data(withJSONObject: params, options: [])
} catch {
return
}

Answer 1

You need to send api key in the authorization header like this:
let params: [String: AnyObject] = ["email_address": email, "status": "subscribed"]
guard let url = "https://us10.api.mailchimp.com/3.0/lists/<listID>/members/".stringByAddingPercentEscapesUsingEncoding(NSUTF8StringEncoding) else { return }
let credentialData = "user:<apikey>".dataUsingEncoding(NSUTF8StringEncoding)!
let base64Credentials = credentialData.base64EncodedStringWithOptions([])
let headers = ["Authorization": "Basic \(base64Credentials)"]
Alamofire.request(.POST, url, headers: headers, parameters: params, encoding: .URL)
.responseJSON { response in
if response.result.isFailure {
}
else if let responseJSON = response.result.value as? [String: AnyObject] {
}
}

Answer 2

Okay, I got it. #Sam's answer helped me realise that the URL I was using was wrong, and I needed to add the ListID into that URL. I also changed setValue to addValue, and changed NSMutableURLRequest to URLRequest. I also added request.httpMethod = "POST" Here is my updated, working code:
let subscribeUserURL = "https://us3.api.mailchimp.com/3.0/lists/<LISTID>/members/"
let requestURL = NSURL(string: subscribeUserURL)!
let apiCredentials = "anystring:<APIKEY>"
let loginData = apiCredentials.data(using: String.Encoding.utf8)!.base64EncodedString()
let params = [
"email_address": email,
"status": "subscribed",
"merge_fields": [
"FNAME": firstName,
"LNAME": lastName
]
] as [String: Any]
var request = URLRequest(url: requestURL as URL)
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
request.addValue("Basic \(loginData)", forHTTPHeaderField: "Authorization")
request.httpMethod = "POST"
do {
request.httpBody = try JSONSerialization.data(withJSONObject: params, options: [])
} catch {
return
}