Bill Bulat
Ansys Employee

I'm afraid that PML is only available for harmonic response analyses:

You can truncate structural media in transient analyses with a non-reflective boundary using INFIN257 which are created with the MAPDL EINFIN command:

Note however, that INFIN257 may only be used to trucate pure structural elements, not coupled field ones (SOLID226): I have a MAPDL example that works but am so far unable to get EINFIN to create INFIN257 elements with a command object in Mechanical. If I get that working I'll share images of the project with you (company policy forbides me uploading files to the Forum).