How to select seats on a Garuda ticket with flying with Singapore Airlines?

Flying GA ticket on SQ metal, DPS to SIN in discount economy ( ? fare code). Can I arrange preallocation of seats? GA says no will SQ be able to help with GA booking ref

It should work if you ring SQ and provide them your E-ticket number they should be able to pull the SQ PNR which you can then log into the SQ website to manage the booking