Future Perfect Continuous is used to talk about an event that will be in progress for some time before a second moment in the future. The action that came first is the future perfect (or, past of the future). We can identify the future perfect continuous by its structure.

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The structure for perfect forms is (have + verb in past participle), and structure for continuous form is (verb to be + verb -ing).

Adding these structures (have + [to be in past participle = been] + verb -ing).

Thus, the structure for past perfect continuous is (auxiliary verb 'have' in future = will have + [to be in past participle = been] + verb -ing):

Will have been + verb in -ING

She will have been taking care of the baby for two days until things go back to normal

If things keep up this way, it will have been raining for three days straight

They will have been working for two days when you arrive.

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References:

AllGenius Drive

Cambridge

Exam English