For defining the prestress through a specimen in the finite element model, you can use predefined field option that if you used Abaqus, you can find it as follow:

Load --> Create predefined field

Which software package are you using? and which version?

You can try traditional, and usulally very effective simulation of prestressing by uniform cooling of cable. Temperature you can easly calculate as a function of thermal elongation coefficient, prestressing force and Young modulus.