ℹ️ How to add a secondary account from the Test DC (Data Center)

• TDesktop 2.1.14+ and Unigram
Hold [Alt+Shift] key and right click on “Add Account” button, there will be a context menu > Test Server

• Telegram Mac
Open Developer menu (5 clicks on the Settings icon) > hold Command key and click “Add Account” > Login

• Telegram iOS
Open Debug settings (10 taps on the Settings icon) > Accounts > Login to another account > Test

• Telegram Android Beta
Go to Settings > scroll to the bottom > Switch Backend > Login.
WARNING: You will need to relogin when back to the Production Server by click again on Switch Backend.
(Update: It was removed since v6.3, but Switch Backend once appeared on Beta v7.2.0.2125)

• Telegram WebK
https://web.telegram.org/k/?test=1

Notes:
Test DC is a Test/Beta Server for Developers to test something.
Sometimes new features are deployed there first, so new updates can be tested without affecting the Production Server.