Open MPC-HC64.exe
Go to View
Open Options
Select Player
Select Keys
Find the command you want to assign
Click the current key field
Press the new key or key combination
Click Apply
Click OK
To create a new custom command, use a command that already exists and assign it a new shortcut
If the command is not listed, use an external key remapper or AutoHotkey
Save the settings
Restart MPC-HC if the shortcut does not work immediately