What kind of adapter do I need to connect this external monitor?

Solution 1:

If you register your Mac with iCloud / AppleID and then use the iOS Apple Store app - it shows compatible adapters. Two I know that work well are the Apple USB-C Digital AV Multiport Adapter adapter and another is the Belkin USB-C to HDMI Adapter

• https://store.apple.com/xc/product/MUF82AM/A
• https://store.apple.com/xc/product/HKQ22ZM/A

Monoprice also has great value and quality adapters in general, but I’ve not tested these specific ones yet.

Solution 2:

Your MBP supports DisplayPort. The monitor supports DisplayPort. Put a cable between them, you're done. No need to mess around with adapters.

Something like this from Amazon: https://www.amazon.com/gp/product/B01J6DT070