I live in northern midwest of USA--have propane central heat. Electricity is cheaper here than propane sometimes. To aid in heating front room I have 2 electrical space heater--one with fan,and one oil filled radiator type. Is placing oil filled radiator type near window smarter than near thermostat on wall for furnace? Also-- the place under thermostat has a vent at bottom or wall--will that suck the heat from the radiator type or no?
Your central heating system doesn't work to keep the house warm: It works to keeps the thermostat warm. If the rest of the house gets warm too, that's just a side effect.
If you put a portable electric heater under the thermostat, then the central heat won't have to come on because the electric heater will keep the thermostat plenty warm. But, that electric heater probably won't do much for the rest of the house.
Yes it is best to distribute any heat close to the windows even if its additional. If you place it next to a return air vent it will add heat to the whole system but only a portion will remain in the front room. I would not place anything (hot or cold) close to the thermostat because the set point will be satisfied to soon before the rest of the house has reached to set point.If your central system is adequately sized for your whole house then maybe the air ducts to your front room are undersized or the dampers need to be opened more.