Substitution for Sap ps on Network Activity

Dear All ,

i want to create substitution in project system .My requirement is to create substitution in network activity .Here i have to write ,if proj- PSPID is k* ,then AFVGD-SAKTO will be 43000000. But when i am going to create ,then i am not getting PROJ structure or PSPID field there . please help me for that to create in t code OPSN .

Thanks & Regards

Sambit