The structure for print parameter, is a little more complex than printer name
Read some documentation like Setting Program-Driven Print Parameters.
For immediate print on a specified printer device :
CALL FUNCTION 'GET_PRINT_PARAMETERS' EXPORTING destination = tsp03-padest copies = 1 immediately = 'X' mode = 'BATCH' no_dialog = 'X' report = 'Z_XYZ_REPORT' IMPORTING out_parameters = pri_params EXCEPTIONS archive_info_not_found = 1 invalid_print_params = 2 invalid_archive_params = 3 OTHERS = 4.
Regards,
Raymond