Slitherine

I have a single wide screen monitor
Samsung C49RG94SSR 124,20 cm (49 Zoll) Curved Gaming Monitor (5120 x 1440 Pixel, Dual WQHD 32:9 Format, 120Hz, 4ms)
I cannot get the game to run in windowed mode. I click on a resolution of 5120 x 1440, then nothing changes but a quick blink on the screen.
How can I get the "windowed mode" to actually allow me to "shrink" the window on my monitor?

I didn't find a windowed screen option, just as an other player with a bit of experience.