Action Details
Requirements and how-to articles for Apple's action:

Before you can upload a build of your app to App Store Connect, you must first create a page for it in your App Store Connect account.

You need this to get it done:

The name of your app (maximum of 30 characters.)
You must select whether your app is available for iOS, tvOS or both.
Primary Language
Choose the primary language for your app.
The unique ID for your app that is not visible on the App Store.
Limit User Access (optional)
You can allow certain people/roles from your team to access your app.
Subtitle (optional)
You can add a subtitle to your app (maximum of 30 characters.)
Bundle ID
Enter the unique App ID string which you registered for your app (note this is case sensitive when you refer to it later on e.g. when signing your app.)
Privacy Policy URL
Provide a link to the privacy policy for your app.
App Store Icon
File format: .png (no transparency) Dimensions: 1024 x 1024 pixels (square with no rounded corners) Colour size: sGRB or P3
Made for Kids
If your app is designed for children aged 11 and under, select the "Made for Kids" checkbox and choose the most appropriate age range (5 and under, 6-8 or 9-11.)
Promotional Text (optional)
You can use your app's promotional text to share the latest news about your app, such as sales or newly added features (maximum of 170 characters.)
One or more keywords which are relevant to your app, separated with commas (each must be more than two characters long.) You can provide up to 100 bytes of content. Names of other apps/companies are forbidden, and you do not need to include the name of your app or company as your app is already searchable by these. (Localisable)
Support URL
The support website you plan to provide for users who have questions or feedback regarding your app. (The support URL must lead to actual contact information.) (Specify the entire URL, including the protocol, e.g. (Localisable)
Marketing URL (optional)
The website where users get more information about the app. (Specify the entire URL, including the protocol.) (Localisable)
Version Number
The version number of the current build of your app (this appears on the App Store page.)
The name of the person or entity that owns the exclusive rights to the app, preceded by the year the rights were obtained (for example, 2019 Example, Inc.). (The copyright symbol is added automatically.)
A description of your app which details the features and functionality (maximum of 4000 characters, plain text only with no HTML formatting.) (Localisable)
The age rating for your app (4+, 9+, 12+ or 17+.)
Trade Representative Contact Information (if app is being distributed in Korea App Store)
If you are submitting your app to the Korea App Store, you must provide trade representative contact information in order to comply with Korean e-commerce regulations.
App Screenshots
See "App Store Connect Help - Screenshot Specifications" howto for specifications.
App Preview Video (optional)
See "App Store Connect Help - App Preview Specifications" howto for specifications.
You can assign a primary and secondary category to your app to help users discover it in the App Store.#
You can choose from the following categories: Books, Business, Developer Tools (macOS only), Education, Entertainment, Finance, Food & Drink, Games, Graphics & Design (macOS only), Health & Fitness, Lifestyle, Kids (iOS only), Magazines & Newspapers, Medical, Music, Navigation, News, Photo & Video (iOS only), Photography (macOS only). Productivity, Reference, Shopping, Social Networking, Sports, Travel, Utilities, Video (macOS only) and Weather.
If your app falls under the "Games" category (either primary or secondary) then you can also select up to two subcategories that describe the genre of your game. (Primary category is required, secondary category is optional.)
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How-To Articles
A list of useful links for you:

Apple - App Store Connect Help
Help guide and information regarding Apple App Store Connect.
App Store Connect Help - Screenshot Specifications
Specifications (e.g. size, source, etc) for app screenshots.
App Store Connect Help - App Preview Specifications
Specifications (e.g. format, resolution, bit rate) for app previews.
Link missing here or found errors? Improve How-Tos