UNL T-Mobile plans have Simple Global Coverage built-in. No additional request is necessary in order to use your device.

Unlimited Data: Up to 5GB of high-speed data in 11 European countries (Austria, Croatia, Czech, Germany, Greece, Hungary, Macedonia, Montenegro, Poland, Romania, Slovakia)  and then up to 256kbps speeds** after 5GB is used. Includes phone and Mobile Hotspot data. Up to 256 kbps speed** in 215+ other included countries.
Unlimited Messaging: Includes incoming and outgoing text/picture messages, to/from any of the 215 included countries & destinations, including the U.S. 
Low Flat-Rate Calling : $0.25 per minute

To view country coverage, as well as terms and conditions, visit T-Mobile's global coverage website.

Under the section named "What country or destination are you traveling to?" you will use the check coverage option to see details. Your plan is considered "Magenta". 

Want more data? T-Mobile also offers International Data Passes

$5 - 1 day, 512MB data
$35 -  10 days, 5GB data
$50 - 30 days, 15GB data

Click here to request a data pass.