Trevp

Ok so this may be a self answering question. On our last cruise with Celebrity we spent our on board credit by booking some excursions with our on board dollars. My question is, can you only spend on board dollars onboard ? I want to use the dollars again for excursions but would like to book them now before they sell out rather that waiting till I board in June. I tried to book on the Celebrity app but it wanted to bill me rather than take it from the on board credit. So I have to wait until I walk onboard ?