A window will almost never provide sufficient light to grow any plant successfully from seed. It can be done with some veggies but only short term and with a lot of experience.

'Nadeshiko' appears to be a synonym for Dianthus superbus, a hardy perennial not at all suited to life indoors. It needs to be sown inside but either under glass or using lights. Fertiliser and tea leaves will do nothing to make up for inadequate light. In fact this plant doesn't need or like rich soils being a plant native to open, light, rocky hillsides. Being a hardy perennial it will not produce flowers quickly, probably not until its second year, if it survives.

We don't know where you are located but if at all possible your plant should be outside (after hardening off.) Google 'Dianthus superbus from seed' for further instructions.

Agree. Trying to grow most anything from seed with just window light is a lost cause. Think about it - even greenhouse growers with all their sun exposure often have to provide supplemental lighting. We have a FAQ here about the supplemental lighting indoor plants require to be successful.

Dave