Ensure the data conforms to the standards described in Setting up the PEM client, and that its format is as described in Format of user data. See Sign-on with correct userid and password and Sign-on with new password for examples of sign-on output data in GDS flows.
Basic conversation information and data are contained in the attach FMH, as described in Format of user data. The sign on request attaches a transaction X'06F3F0F1', which is the SNA service transaction program name for the sign-on transaction program.