There is a custom among Smarthas to apply that ash on the forehead after performing Samitadhana homam. That custom does not exist in the Sri Vaishnava tradition. Furthermore, I recall seeing that there is a specific mantra for Sama Vedis to apply that ash. For them, it is an exception because there is a mantra. It is not so in the Yajur Veda. However, Yajur Veda Smarthas do apply it. It is not applicable for Sri Vaishnavas.