How to set BC on free edge on specific plane using .tcl?

Hello, I would like to set boundary conditions and a torsional moment on free edges of a specimen using a .tcl script. Creating the BC's manually and checking the command.tcl document does not give the desired result. For example, the command.tcl document uses

*createmark nodes 1 1-7
*loadcreateonentity_curve nodes 1 3 1 0 0 -999999 -999999 -999999 -999999 0 0 0 0 0

to create a BC at the lower free edge of the specimen. 

The geometry of the specimen should be slightly modified to compare different variations. Because of these changes I am looking for a similar method to create BC's not on a list of nodes but on a free edge on a certain plane (e.g. on all nodes on the x-z plane with y=0).

Thank you very much for any advice.