italki – How to say “at your earliest convenience” but in a more demanding way? I am writing a letter to the

How to say “at your earliest convenience” but in a more demanding way?
I am writing a letter to the Client, so I have to stay polite but still remind them that they have to provide me the details else my hands are tied up:
1. How to say “at your earliest convenience” but in a more demanding way?
2. How to remind gently about Client’s obligations? (“Kind reminder” does not sound really good to my ear in this case).

With regard to postponement of a specialist mobilization for [works…] till September, please specify the date and period at your earliest convenience [later when you know, because right now I am sure it’s not possible to say]. Kind reminder that we need to send the request to the specialist 3 weeks in advance.