If she gets fresh greens you could maybe squirt some on the veg before you serve them? Vitamin B tends to have a pretty strong taste though, so try with just a little bit for starters.
It is stressful for most rabbits to be restrained and force fed, so that’s obviously not very good unless you absolutely have to give meds or force feed. If you know what’s in the supplement, maybe you could get an edible one with roughly the same contents? Oxbow have vitamin supplements in the form of chewable tablets, I’m sure there are other brands as well.
Rabbits produce their own vitamin bs in their intestines with the aid of their gut microbes. This requires a sound and sturdy gut microbiome (bacteria and fungi, like yeasts, that live in the gut and help break down food into usable micronutrients). A rabbit probiotic can help keep the gut microbiome in good order so the bun can produce enough vitamin b on its own. As for prebiotics (the food the gut microbes live on), hay is the absolute best one.