People have been requesting the ability to change modes through IFTTT for two years, but they haven’t added it yet.
If you have the SmartThings Classic app, this is easy to do by creating a virtual switch, then creating a Routine to change the Mode based on that switch coming on. The same method is used if you want to change a mode with Alexa or Google Home.
We have a FAQ that explains further. This is a clickable link.
I don’t know how you would do it with the SmartThings (Samsung Connect) mobile app.