Wiz Khalifa & Amber Rose Flirting with a MARRIAGE LICENSE...

The countdown to the wedding IS ON ... 'cause TMZ spies confirm Wiz Khalifa and Amber Rose were both at the marriage license kiosk at an L.A. courthouse today.

If all is as appears, the couple has 90 days to get hitched before the marriage license expires.

Amber -- who just gave birth to the couple's first son -- has been calling Wiz her "husband" for months ... but unofficially.

The two have been discussing marriage for a while, but have yet to reveal exactly when and where they plan to tie the knot.

So far, no word from Amber's camp.

Congrats in advance!!