Adverb: Gladly, willingly, not grudgingly, with pleasure.
The basic form of this adverb is miellelään. In our example phrase, miellelään is in the 1st person plural form, mielellämme so the form of the adverb is in agreement with the subject of the sentence. In this case, it is 1st person plural because annamme means ”we give.”
The whole phrase annamme mielellämme lisätietoja means ”we gladly give more information.”