I'd have to experiment with a test case to be absolutely sure this works, but I'd start by making sure that you save calculated nodal forces to the results file and set "Save MAPDL db" to "Yes" before solving. These settings are made in Analysis Settings Details.
Then, in a post processing command object, you could select the pretension elements and list forces using PRRSOL of PRRFOR:
The reactions force listing would appear in either the solve.out or post.out file, depending on whether you run the create the post processing command object before or after solving.
Similarly, you could try this (if it works it'll show the net bolt load force in Details of the command object):