助走つける take a run up
助走つけてやるもの something you take a run up for
助走つけてやるもんじゃない not something you need to take a run up for
つける means to “add” the run up for whatever it is you’re taking the run up for
In this case it seems to be that he’s asking someone to help stretch by pushing his back. They put too much effort in so he says “you don’t need a run up, for stretching”
Look on YouTube for examples of 助走