Using Johnrts air suggestion would probably work fine but I would just hold the glue bottle tightly up against the MDF and give it a good squeeze in a couple spots. Glue will go in and then more glue will push the glue deeper. Wipe off the excess before you clamp and then wipe off what comes out after you clamp. Squeezing the crack shut when clamping will likely squeeze most of the glue out because you really only want a very thin layer.