What cable to connect 13-inch Macbook Pro 2018 and 34-inch 3440x1440 monitor? I have a Macbook Pro 13-inch 2018 edition with four Thunderbolt 3 ports.
I've just bought an Iiyama 34-inch 3440x1440 monitor. It has two HDMI ports and one DisplayPort. What cable should I buy to connect the two?
I'm considering this DisplayPort-Thunderbolt cable, but it says the supported resolution depends on the device and I'm not sure how to check whether the Macbook will support it.
Your Mac can easily handle a 3440x1440 monitor (in terms of resolution), using Thunderbolt-C to DisplayPort as linked in your question.
For information about the specifications of your computer, see Apple's Website.
You can have one monitor at 5120x2880x60Hz (billions of colours), up to two monitors at 4096x2304x60Hz (millions of colours), or up to two displays at 3840x2160x60Hz (Billions of colours) 
Also, the MacBook Pro is listed as a compatible device on the listing for that cable