Hello,
from your question it is not clear if you have
a) pre-tensioned bars/wires - concrete cast over wires fixed outside, as used, e.g., for railway sleepers
b) post-tensioned cables - cables drawn through ducts, then tensioned, then either
ba) only kept fixed at ends or
bb) bonded by injecting concrete or some adhesive
c) some other situation

To be able to give concrete advice, we need to know which ATENA module you are using for the modelling - ATENA Engineering 2D, Engineering 3D, or ATENA Science/GiD (or at least which of them you have and can choose)?

Hello, in this case, applying initial strain to the reinforcement is the best way. In ATENA Engineering 3D (unlike ATENA Science-GiD), initial strain can not be defined directly for reinforcement bars. I recommend the following "trick": apply cooling (i.e., negative relative temperature) to the prestressed wires. Set the thermal expasion ratio and the temperature to get the right amount of prestressing in each wire.

Hello, I am not sure if I understand the question?
To display the resulting stress in the wires, just display 1D results and select the stresses (X direction) for display.
To calculate the right amout of "thermal" strain to be applied, just divide the stress with the reinforcement elastic modulus (of course, if you have to consider prestress losses due to shrinkage, and do not wish to apply shrinkage as a separate load, youhave to reduce the calculated pre-stressing strain to reflect the losses).