Ordinary linkedit will pick up assembler subroutines invoked by IBM "CALL" macro.
To invoke a dynamic subroutine, invoke IBM macro LOAD (check out the parameters), then simply branch to the first byte of the loaded subroutine loadmodule. Upon completion the subroutine will return control to the calling module.
You will need to GETMAIN to ensure storage is available for you dynamic subroutine. You can load your subrtn below or above the line. The LOAD will be executed once only, before the first execution of the subrtn.