Transformers 🎬 Optimus Prime Flying scene....

After Optimus was able to aquire a new chassis in the desert in Age of Extinction, I presume he was also granted the ability for rocket boosting himself into space. Using his shield to deflect the hazards of space travel.
However he didn't get very far… as we learned in The Last Knight. He most likely ran out of energon and went into stasis. Quintessa directed him into Cybertron and the impact of reenter shocked him out of stasis.