The future perfect tense is when the action will have already been done at some point in the future. It is denoted by the use of "will' + "have" + the past participle form of the verb.
So, the sentence provided in the question
They willhave<u> finished</u> (past participle form or third form of the verb 'finish).
is in the future perfect tense.
Therefore, the presence of "will' + "have" and the subsequent third form of the verb "finish" clearly shows that it is in the future perfect tense form.
