I THINK IT IS WRONG

I  will have done  this before that happens

IN THE EXAMPLE OF FUTURE PERFECT

Hello,

The sentence mentioned in your query is correct and shows the correct usage in future perfect tense.

Here are a few more sentences in the same context for your better understanding:

• I shall have completed the task by then;

• He will have left before you go to see him;

• By the end of this month I will have worked here for five years.