[Unable to offer designated player contracts]

To offer a designated player slot you should offer them "star" playing time

[No preseason]

This can be recreated by starting a career with an A-League team. The earliest option is late pre-season. By then, the transfer window has shut and the first rounds of the FFA Cup have already happened.

[A-League Squad Registration]

Yep this has been an issue in FM forever, 

I don't complain though it means I get the best players for free.

 

Just look at the released players at the end of each transfer windows, especially the first one.