According to the 4th edition of the Microsoft Manual of Style, Spacebar should be capitalized, similar to the other keys on the keyboard.
Several general language dictionaries reference the word without capitalization, written as two words (cf. Oxford Learner's Dictionary, Merriam Webster).
To make matters more confusing, Microsoft's Terminology Portal proposes several different variants: SPACEBAR, Spacebar, and spacebar.
A search in Google Books provides yet more examples illustrating why we're all asking ourselves this question; SPACEBAR, Spacebar, space bar, and spacebar are all found in publication.