Everything that Jacqui said; in addition:
– You can also blow the moisture out of the windway by taking off the top joint, covering the end with your hand and blowing into the window. If you open your mouth and cover the window, you don’t touch anything at all so no possibility of any damage, and it has the benefit that you blow the moisture completely out of the instrument rather than further down it.
– You can improve things by using soapy water in the windway to reduce the condensation. There’s lots of info about this if you google it. A plastic instrument can literally be soaked in soapy (washing up liquid) water and allowed to dry. Alternatively, I use Bernolin’s anti-condensation flued which seems expensive but a little goes a very long way and I have found that it works very well, better than the soapy water. A small caution is that it makes a slightly white-ish mark when it dries so with a wooden instrument it’s best not to spill too much of it.