In turkish there is a character "ı" which is used frequently. The only way I know of putting it into a text field though is copying it over from somewhere else. I'm assuming the answer has something to do with typing in the correct Option+[...] keys to compose it, but I have no idea what the magic combination is.
One source I found said it was ⌥⇧B (that is,
If you want to find out where all the symbols are on the keyboard, go to System Preferences - Keyboard and enable "Show Keyboard Viewer in menu bar". Then click on the new symbol in the upper right menu bar and select "Show Keyboard viewer".
Now you can hold down ⌥ and/or ⇧ to instantly see all possible characters.
You can also use Ukulele to edit your keyboard layout in case you need to modify it.
It varies by keyboard layout, of course.