You can model and bind your character in whatever pose you wish. However, HumanIK wants the rig to be put in a t-pose for control generation (the skeleton definition). All you need to do is manually rotate the joints so that they are roughly straight and world-axis oriented (the interface for HIK lets you know when the joint chain is acceptably straight, etc). I like to bind the skeleton on frame 0 and key all the t-pose rotations on frame 1. Make sure you lock the definition when you’re done the control setup.