olemus
mingi firma toodete ja teenuste propageerija
= an external promoter of a brand

(a) tasuline, eelistatavalt tuntud isikute hulgast
= employed one, preferably a celebrity
https://en.wikipedia.org/wiki/Brand_ambassador

(b) vabatahtlik,
= a voluntary one
https://socialladderapp.com/2020/12/15/from-voluntary-cheerleader-to-growth-driver-a-beginners-guide-to-becoming-a-brand-ambassador/