Type [ for þ, ] for ð
Guess the five-letter word in six tries. After each guess the tiles change colour: green for the right letter in the right place, gold for the right letter in the wrong place, black for a letter that is not in the word at all. Keys you have not tried yet stay grey.
Þ and ð are letters here. Wherever English writes a th that is really one sound, Thorndle writes one tile. So worth is WORÞ and too short to play, while worthy is WORÐY and fits perfectly.
Which of the two you need is a matter of sound, not spelling. Say the word: if you hear a buzz, it is ð (mother, rather, smooth); if you hear a hiss, it is þ (thanks, health, throat). If it is neither, as in the word "pothole", just write it as usual.
On a physical keyboard, type [ for þ and ] for ð. The þ button in the top bar switches the violet highlighting on and off.