OK, you got a Plumeria seedling and you want that first flower.

First optimize the basics, soil(good-great drainage), sun (6-8 hours direct or more if possible), water( let it somewhat "dry out" between waterings).

Fertilizer, can be a controversial topic! Don't worry, Plumeria respond to a wide variety of fertilizers. I go very light on the fertilizer until I see more leaves and more rapid growth along with heat (temps getting over 80). Organics, I'm a fan. Generally organics work more slowly and have less salts & consequently are less harsh & less burn than chemical based fertilizers. Organics keep your soil alive so bugs and microorganisms are able to exist and break everything down to basic nutrients the plant can use. During the prime growing season Plumeria are heavy feeders. Chemical ferts. already ARE the basic nutrients your plant requires. They have their place from general application to foliar feeding to slow release granules. Along with the organics mulching can be even more appropriate, for moisture retention and a more lively organic "culture". If bugs bother you, don't do organics...do chemicals. If you dare mix organics and chemicals, you have to be careful to keep the chemicals way way down 10-20% max, 80-90% organics ratio, so you don't kill the soil.

Put the basics in place, watch the plant grow, then let nature take it's course. Not counting my stragglers, I'm fortunate enough to have 3 out of 30 plumeria seedlings giving first flowers at three to four years from seed...roughly 10%. Odds are 15-20 of the rest of my seedlings will be blooming in years 4,5&6. Good things will come....

Good advice above. The best thing to do is keep a routine and I prefer to use half the recommended dose of fertilizer twice as often. Thus keeping a steady supply of nutrients.

I don't use bloom booster types but you can decide for yourself what works. Please be wary of too many salts building up in the container soil. Eventually there will be a point of diminishing returns.

@the_first_kms2 what brand of fertilizer do you use?

If it was grown seed, it may be a few years to bloom despite your fertilizer efforts.