How can I fix the temperature in a concrete cube ?

I try to learning OpenStudio software by building a simple concrete cube. I did calculation by hand for compare both results with Openstudio results. But when I wanted to fix the inside temperature of the cube I didn't know how to do it. I think I should add an HVAC system in the cube in order to fix the temperature to have the same results as the calculation by hand.

Can someone explain to me how can I do this and a quick explanation of installing an HVAC system in my cube?

You don't have to simulate an actual HVAC system for your purpose. It is really different if you are using OpenStudio or EnergyPlus. In case you intend to continue your simulation with OpenStudio, after defining object in Schedule, Construction, Loads and some unnecessary tabs, in the Thermal zone tab you can use Idea Air Load:

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