Spell it however you feel is closest to the way it sounds in Arabic. However Allah is a proper noun which should start with a capital. The following would all be acceptable:
InshaAllah
Insha-Allah
Insha'Allah
Insha Allah
There is a correct way of spelling it.
Its actually three (3) words in Arabic
Inn (which is a Sister of Inna)
Sha'a
Allahu
The word Inna in arabic makes a word mansoob and literally means "INDEED, DEFINITELY, CERTAINLY".
Al-Insaanu = The Human
Inna Al-Insaana = Indeed! The Human
Where-ever you see Inna in the Qur'aan, Allah (SWT) is FORCEFULLY & EMPHATICALLY making a point!
so of course this must be expressed correctly in our way of spelling it...
it might be that ebsoon is not familiar to these rules and of course geographic locations may also make a difference....
Just thought to clarify!