does anybody have any tips to GAIN weight?

im (19F) currently pretty underweight, 173 cm and 47.6 kg (which is around a 15.8/15.9 BMI). these past few years ive been losing hair like crazy, been so drained of any energy that i cant get out of bed, and i have no boobs at all. i always thought it was a mental health thing or im just genetically super flat (which could be the case but still). im just starting to piece together that its probably my weight, which ig is late but ive been this weight practically my whole life so its normal to me.

anybody have any tips on how to gain weight? id love to reach a bmi of 18.5 by september. thing is i have no appetite AT ALL, most i eat is probably one meal a day and im fine. its really hard to force myself to eat, so if anybody knows of any supplements i can take for the time being or any tips in general it would be really appreciated. (and im really hoping to grow my chest too 😭 literally a 32AA cup rn)