I just checked my TK M&S ticket again on TH.com and seat selection (C class) was still easily accessible under "Additional services"
You will get the following warning "your request cannot be completed by the application. Please contact your travel agency or Thai Aiways office. (1017 - 3002)but just ignore it; you seat selections have been confirmed already.
Last edited by SK AAR; Jan 14, 2018 at 4:45 am