When you want to find the on ledger unique transaction identifier (Transaction ID, also known as transaction Hash), you can open the transaction at the Events tab in XUMM.
If you swipe down, you will see the Transaction ID field.
Alternatively you can copy the Transaction ID to be viewed at a XRP Ledger blockchain explorer by tapping the "three dots" icon on the upper right corner of your transaction detail screen in XUMM (after opening the transaction at the Events tab).
You can use this Transaction ID to verify that XUMM did indeed send the transaction on the XRP ledger, and the your XRP has in fact been credited at the destination account.
You can use the Transaction ID / explorer link as proof of the transaction when sending a support request to an exchange.