Following on from Shiva's response, the key question here is why would you want to use the ATP check if the requested quantity is always confirmed?
There is no way (other than RLT) to do this in the standard system. You would have to use EXIT_SAPLATPC_002. Here, you can fill P_MDVEX-MNG02 (confirmed quantity) with the P_MDVEX-MNG01 (required quantity). This will overwrite the standard result of the ATP check.