The story
Written form of a reduction of want to be, analyzable as wanna + be. Wannabe can be considered a conversion, category change, or functional shift. While occasionally appearing in print, usually as a less reduced phrase, the popularity of the word took off in the mid-1980s following the Madonna wannabe fashion trend: “wannabe”, in Google Books Ngram Viewer.
Etymology from Wiktionary, CC BY-SA 4.0.