It doesn't sound like they want that -- they probably just want to look up the status of the invoice you paid
You might be right. I can't tell for sure.
In this post I had written "Payment hash", but in looking back at the message I received from their support the term they used was the "Transaction Hash", and not the "Preimage". So you are probably correct -- they were likely looking for the (publicly available) "Payment Hash". I had previously already shared the LN Invoice with them, so I was assuming they needed something else and then assumed they wanted me to give them the Preimage (proving I had paid it).
And thanks for the reminder about LightingDecoder.com, I had forgotten about that so even if I did need the Payment Hash, I doubt I would have known how to obtain that, if I used only WoS, for instance.
Decode Lightning Network Requests - BOLT11, LNURL, and Lightning Address #68078 https://lightningdecoder.com
I also now see:
bolt11 invoice interactive explorer #68012 https://www.bolt11.org