In American English you don’t pronounce it.

I can address North American phonology in detail, but not British phonology. [j] is not standard in the pronunciation of the word immediate. Source: New Oxford American Dictionary - immediate [ ɪˈmidiət ] Source: Oxford Learner's Dictionaries web site - immediate [ɪˈmiːdiət] (AmEng) If a speaker were to insert [j], it would normally assimilate with the preceding [d] and become [dʒ]. I have heard regional pronunciations of "immejit" [ɪˈmidʒət]. The schwa is a (variable) central vowel. It varies (assimilates) depending on the phonetic environment. Here [ə] becomes [ɪ]. So, immediate [ ɪˈmidiət ] > [ ɪˈmidiɪt ]. A [j] doesn't normally occur as the tongue transitions from [i] to [ɪ] to [t]. Another example of schwa becoming [ɪ]. Oxford Learner's Dictionaries web site - wanted [ˈwɑːntɪd] (AmEng) Given the [t] before the schwa and the [d] after the schwa, the schwa becomes [ɪ]. Here is an example of [i] becoming [j]. Source: New Oxford American Dictionary - California [ˌkæləˈfɔrnje] In brief, the final vowel is a schwa, and the sequence [niə] reduces to [njə]. Curiously, the (British) Oxford Learner's Dictionaries site gives California [ˌkæləˈfɔːrniə] (no [j]) but has an audio recording of a native speaker with an easy-to-hear [j] (which is correct, I am from California).

When pronouncing "immediate," the 'y' sound between "di" and "ate" is typically not pronounced. Instead, it's more common to blend the two syllables together smoothly, saying "im-me-diate" rather than emphasizing the 'y' sound. This makes the pronunciation flow more naturally. However, regional accents and individual speech patterns may vary, so some people might include a subtle 'y' sound, but it's not necessary for standard pronunciation.

/ē/mēē/dēē/ĕt