Contactless travel is not like contactless payments in a shop. When you pay for something in a shop, you and the shop both know how much you are going to be charged, and so you can insist on a receipt showing that amount. When you touch in and out on a ticket gate, how much you will be charged will depend on what other journeys you have already made that day and week, and potentially what other journey you are just about to make if it can be counted as a continuation of your journey. So you can't just go to a ticket machine and get a receipt for the journey that you have just made.
If you are going to be doing this regularly, then I agree with the suggestion about setting up an account so that you can get a PDF of your journey history for the relevant day(s). For a one-off, it's probably simpler to use your card to buy a conventional ticket (this may cost more, but you may not be worried about this if you are claiming the cost back on expenses).
It may be worth checking if your employer has a policy for this. One company that I worked for insisted that if you are going to claim for Oyster/contactless travel, then you had to set up an account and submit the journey history. They would not accept a bank statement, as it did not identify the journey in sufficient detail for the tax man.
If you are going to submit a bank statement as evidence for your expense claim, some banks allow you to produce customisable statements on-line where you specify the start and end date, if you don't want to give your boss full details of your month's outgoings.