Is that not a feature that Let's Encrypt supports?
The TXT filed will contain a challenge code to be put into the TXT field. This code is give to the acme script by Letsensrypt. For example : 'bmDWOCHFZRtOOCr_vU-mEfTIqA6i9ib0R3V6-RMF3FE'.
This bmD....................RMF3FE thing is generated randomly, and will be unique for every certificate request.
This proofs that you control right now - and not some time X in the past.
Note that, ones this test passed, it stays valid for one week.