It is possible to assign a hotkey or chord (multi-key) to command Spell as you type to toggle this feature by key. Open Advanced - Settings or Configuration - Key mapping, scroll down in Commands list to Spell as you type and select this command, set with a mouse click the caret into field below Press new (multi-)key, hit the key combination to use in future to toggle this feature and click on buttons Assign and OK to set this hotkey assignment and exit configuration with saving modified key mapping.

It is true that command Spell as you type is available neither in ribbon nor in toolbar customization dialog. So this command cannot be easily added to a custom ribbon tab or a customized toolbar although I suppose it would be possible using a method similar to Manual customization of command OEM Character Set. Therefore I suggest to write an enhancement request email to IDM support which requests adding command Spell as you type to ribbon and toolbar customization dialogs to be able to add this command to a custom ribbon tab or a customized toolbar as it was possible prior UltraEdit for Windows v23.00 introducing the ribbon interface. I have already done the same. The more users request an enhancement the higher becomes its priority for implementation.