You cannot claim for costs in advance, only costs you have already incurred.
If you’re on a long-term placement you can claim part of the costs before the end of the placement if you prefer. This means, if you are on a 2 month placement you can submit a claim for costs from the first month once you have paid them. You can then submit another claim at the end of the placement for the second month.
If you choose to claim in this way, you must make sure you do not claim for overlapping dates. The system should stop you if you try to claim for dates previously claimed.