Please turn the breaker off unless verifying the hot wires.
Both are hot because this is 240v I found several Mysa thermostats so you will need to provide the model #. Here is how 240v baseboard heaters work (there are 2 possible wiring combinations.
240v hot on black to red 1 switch contact set or a single pole single throw switch: the hot wire usually the red is switched you do this by identifying the hot wire and put this on the L or L1 terminal or wire the other 2 going to 2 heaters? Are on the other contact. If only 1 heater and this feeds another thermostat you need to figure out witch goes to the heater and the one going to the next thermostat, the one that goes to the next thermostat goes with the hot on the L or line terminal, the one that goes to the base board may be a T or output terminal (if only 2 screws the one not used) . All 3 blacks tied together
This will control the power to the baseboard if the thermostat doesn’t require a neutral. Standard single mechanical switches work this way.
The negative with this kind of switch is that there is always 120v going to the heater but it will only heat when turned on.
If your thermostat is a double pole single throw or 4 contacts you still connect the red the same way. But you now connect all the blacks similarly to the red bit on the other side not in use , black hot should be on the L 2 or line 2 screw or wire the base board in the room should be connected to the terminal that is left.
The hot cable 1 wire on each side of the switch, to the baseboard each on the other contacts.
Do not connect red to black that will be a direct short when the switch closes.
The advantage of a double pole switch it can control both hot’s for 1 baseboard or control 2 baseboards with 1 switch 1 baseboard on each side so I guess that would be a 3rd way to connect them up.
Now turn the power back on and if the thermostat is 240v no neutral it will work.
If the myasthenia has a white it will not work as you have 2 Hot’s and a ground.
This is normal for 240v baseboards.