Rule one: only a registered tax agent may say so
The FTA states that it is prohibited to practise as a tax agent without registering and being accredited by it. A tax agency, the firm, may not practise until at least one registered tax agent is linked to it (FTA, Registration of Tax Agency). The bar to register is real: a relevant degree or certification, three years of experience, Arabic and English, the FTA exam, professional indemnity insurance and an AED 3,000 fee renewed every three years (FTA, How to become a tax agent).
So FTA registered tax agent is a claim to make carefully and precisely. Use it only where the register shows it, put the accreditation number beside it, and do not stretch it from one partner to the whole firm. The FTA's own guide says it is not its role to promote any tax agent, so registered with the FTA is a fact and recommended by the FTA is not.