Top dress best. Highly soluble and mobile, will wash out fast. Careful overdoing it though. Potassium toxicity, especially moderately will decrease quality, and yeild. Very dark green leaves...You would think they should've done better.
Please remember we are helping based on a picture taken under grow lights. We can't clearly see any obvious chlorosis in your picture.
Dont do anything yet!!
Do your research take my info and others with a grain of salt and dont over think it and dont over do it.