Hi Bubu, sometimes we translate “frappucino” just according to its pronunciation directly, so it’s “fǎ bù qi nuò”. But it’s also called “xīng bīng lè” （星冰乐）. And “lǜ chá” is “green tea”, so here comes:

Wǒ xiǎng yào yī bēi lǜ chá fǎ bù qi nuò.

or,

Wǒ xiǎng yào yī bēi lǜ chá xīng bīng lè.