Is it permissible to open a dummy account to earn the discount offered by the fuel company?

If the company's system does not prevent subscription under a fictitious name, then there is no harm. However, if the company does not permit this, then circumvention of its terms is forbidden because it involves deception, fraud, and unlawful acquisition of wealth. This is based on the Prophet's (peace be upon him) saying: "Whoever cheats us is not of us," and in another narration: "Whoever cheats us is not of us; deceit and trickery are in the Hellfire."