Peace be upon you. Saying 'Wa Ajil Farajahum' after the Tashahhud does not harm the validity of the prayer. According to Grand Ayatollah Khamenei and Grand Ayatollah Sistani (may Allah protect them), since this phrase is a form of supplication, it is permissible to recite it at any point during the prayer without intending it as a specific part (i.e., as a recommended act). May you be successful in all good.