Because it’s a conjugated verb. You need to look up the infinitive: booster, for which Grand Robert & Collins offers

booster (2)

[+ économie, ventes] ▶ to boost

[+ moteur] ▶ to soup up

Collins Dictionaries Online doesn’t have it (it has booster, the English substantive; la fusée de lancement). It’s a fashionable loanword from the English language but it is poorly compatible with French in terms of spelling and phonetics. Alternate word choices from the BDL :

gonfler, augmenter, stimuler, encourager, accroître, développer,
renforcer, relancer, animer, hausser, dynamiser, doper (langue
familière), donner du tonus, donner (ou redonner) de l’élan, donner un
coup de fouet, donner un coup de pouce, remonter le moral, faire du
battage, promouvoir, valoriser, faire la promotion, faire monter.

And from the Académie française:

Stimuler, régénérer, relancer, augmenter, accélérer, développer, faire
croître.