We usually say we work "at" or "for" a company--both are common.
Use "since" and "until" alone, or use the pair "from...to"
Mr Black has been working at/for this company since April 25, 2018.
Mr Black has been working at/for this company until now, but he is considering leaving it.
Mr Black worked at this company from 2015 to 2016.
Mr Black has been working at/for this company from April 25, 2018 to the present.
Like Rena, I prefer the first choice:
Mr Black has been working at/for this company since April 25, 2018.
the present perfect continuous tense makes the "until" portion redundant.