Learn Dutch: Ik kies deze behandeling en laat mijn baard reinigen. - I choose this treatment and have my beard cleansed.

The verb laten is used here as a causative verb, which means you arrange or pay for someone else to do an action for you. Just like in English you 'have your car fixed' or 'have your hair cut', in Dutch you use laten plus the action verb. You aren't doing the cleansing yourself; the barber or specialist is doing it for you.

Why is reinigen at the very end of the sentence?

This is a classic Dutch word order rule. When you have a main conjugated verb (like laat) combined with a full infinitive verb (like reinigen), the infinitive gets pushed to the very end of the clause. You literally say that you 'let my beard cleanse'.

Why isn't there an ik before laat?

Because the subject ik (I) is exactly the same for both halves of the sentence, you can drop it after the connecting word en (and). The single ik at the start safely applies to both kies and laat.

Why do we use deze instead of dit for 'this treatment'?

The word behandeling (treatment) is a de-word (de behandeling). For all singular de-words, you must use deze to mean 'this'. You only use dit for singular het-words.