What is the difference between installing a humidifier before a heater and after a heater?
warm air can hold more moisture than cold air so the amount of water you can suspend in the air will change depending on where you install the humidifier.
Mounting the humidifier before the heater will give you a final relative humidity below 100% after the heater. This can be important if you want to avoid dew.
Mounting the heater before the humidifier will allow you a higher absolute humidity after the setup.
There's a secondary consideration: temperature. It takes a lot of energy to convert liquid water to gaseous water. If your proposed humidifier is not generating steam (gaseous water), then wherever the humidifier is placed, the air will be cooled as the water or water mist evaporates. If before the heater, and the heater actively manages the output air temperature, then you won't see an effect. If after the heater, the temperature of the air sent to the house proper will be lower, and it'll take longer to reach the design temperature.