Involved in the carboxylation of phenylphosphate. Flavin prenyltransferase that catalyzes the synthesis of the prenylated FMN cofactor (prenyl-FMN) for 4-hydroxy-3-polyprenylbenzoic acid decarboxylase UbiD. The prenyltransferase is metal-independent and links a dimethylallyl moiety from dimethylallyl monophosphate (DMAP) to the flavin N5 and C6 atoms of FMN. dimethylallyl phosphate + FMNH2 = phosphate + prenyl-FMNH2 Belongs to the UbiX/PAD1 family.