The 'Wings' hitmakers - who shot to fame when they won UK TV talent show 'The X Factor' in 2011 - are delighted the US star agreed to perform a guest rap on 'How Ya Doin'?'.
Band member Jesy Nelson said: ''We honestly can't believe that Missy is on our song! She's been one of our biggest idols for ever - it's incredible.
''As soon as we thought there might be a chance she'd appear on the track we did literally everything we could to make sure it happened.''
Meanwhile, the group - which comprises Jesy, Leigh-Anne Pinnock, Jade Thirlwall and Perrie Edwards - are preparing to launch their careers in America and have thanked their new US fans for their support.
Jade told MTV News: ''They're just incredible, aren't they? As soon as we got here, there were fans at the airport waiting for us with presents, balloons and at the hotel.
''It's just everywhere we go, we've got a good little selection of fans. They're so polite.''
Perrie added: ''Our fans, like all over the world, like ever Little Mixer we have, are crazy in a good way. It makes us feel really good.
''They make you feel so welcome and I think as well, with our Littler Mixers, they're so dedicated. They are just proper hardcore and that's what we want.''