Here is the Amazon links to the contactor and app controlled switches that I used.

Sonoff WiFi Switch 8-Pack Alexa Switches Wireless Remote Control Electrical for Household Appliances,Compatible with Amazon Alexa and Google Home,DIY Smart Home Devices on Your Phone https://www.amazon.com/dp/B078S21X9T/ref=cm_sw_r_apan_glt_fabc_7P3F5VTY07XSTN6E4RZR?_encoding=UTF8&psc=1

Packard - DP301202 C230B 2 Pole 30 Amp Contactor, 120 Voltage Coil https://www.amazon.com/dp/B001KGSJ74/ref=cm_sw_r_apan_glt_fabc_BWDVZB8MPX02EGB7AGWD?_encoding=UTF8&psc=1

I dont have a pic or schematic handy but basically you power the app switch with 120v and then that connects to the coil of the contactor. When the switch is engaged, it makes the contactor turn on. Then you just run your 240v in one side of the contactor and out the other. May be able to google some wiring diagrams.

Second this. I have the same setup. I considered circumventing the timer, but decided against it for safety and energy savings protection.

Hardware list for a 6kW heater: (As AudioVoodoo mentioned, size amperage ratings for your application accordingly)

IF YOU HAVE ANY DOUBT, CONSULT/HIRE AN ELECTRICIAN

Packard - DP301202 C230B 2 Pole 30 Amp Contactor, 120 Voltage Coil Amazon link

Smart Light Switch Apple Homekit - Refoss Single Pole Smart Switch Wi-Fi Wall Switch Compatible with Siri, Alexa & Google Assistant, Remote Control Amazon link

Ogrmar Plastic Dustproof IP65 Junction Box DIY Case Enclosure (8"x 6"x 4") Amazon link

TayMac MM7440C Two-Gang Weatherproof in-Use Cover, 55-in-1, 2, Clear Amazon link

GFCI Outlet 15 Amp, Tamper-Resistant, Weather Resistant Receptacle Indoor or Outdoor Use, UL Listed, LED Indicator with Decor Wall Plates and Screws Amazon link

Edit: Links only for convenience, no affiliation to Amazon or any of the products.