FORTNITE 

To find your unique Epic Games account ID in Fortnite, open the game, go to the main menu, and select Settings. Choose the Account and Privacy tab, then hover over Epic Games account ID. Your ID will appear on the right side.

ROBLOX

On a Computer
Open a web browser and go to Roblox and log into your account.
Click the three horizontal lines in the top-left corner and select Profile.
Look at the URL address bar at the top of your screen.
Find the long number between users and profile. That number is your Roblox User ID.

On a Mobile Device
Open a web browser like Safari or Chrome and go to Roblox.
If prompted to open the Roblox app, choose Continue in browser.
Log into your account if needed, then tap the three horizontal lines and go to your Profile.
Tap the URL address bar at the top of your browser. 
The series of numbers in the web address is your User ID.

DIABLO IV GOLD

In-Game 
Open your game menu (Press Esc on PC, or Options/Menu on consoles).
Look at the top-right corner of the screen.
Your BattleTag (Name followed by numbers) will be clearly displayed there.

Battle.net App 
Open the Battle.net app.
Click on your profile name in the top-right corner.
Your unique account number (which differs from your text name) is listed next to your BattleTag in the drop-down menu.

FC 26

On Console & PC
Open the game.
Click the Settings (cogwheel) icon in the top left corner.
Scroll over to the Online Settings menu.
Select EA Account & User Agreement (or Privacy Policy).Y
our EA ID/Username and the email linked to your account will be displayed on the screen.

On the EA App
Open the EA App on your computer.
Click on your profile icon in the top right.
Select Settings, then click My Profile.
Click the Manage button.This opens an external browser. Once you log in, your EA ID and Account Information will be listed there.

On FC Mobile
Open the game.
Tap the Settings (gear) icon on the top right.
Scroll down the settings list to find UID.
Your User ID is the sequence of numbers listed there. You can tap the overlapping squares next to it to copy it to your device's clipboard.
For more information on specific products, click here: https://www.kinguin.net/campaign/in-game-items