Need to invest 400k now for 6-12 months in liquid securities how do I get the best return

Hi, I received 400k which I have to return in 6-12 months on relatively short notice (tax). The problem is that I dont know exactly when it needs to be returned and it can be on short notice (2-4 weeks). TBH pretty desperate now what to do with this. Can someone who is into liquid investments maybe help with some recommendations?
I would like to invest in secure, liquid instruments but would like to get a decent return on this as well.