You must ‘register yourself as a drone operator with the National Aviation Authority of the first EASA country you intend to operate from’.
For example, if you are travelling to Spain first to fly your drone, you would choose this country as the one that you intend to operate from.
You cannot change the host country that you have registered your operation in once the registration is complete. However, you can then operate in any EASA country!
What countries are classed as an EASA country?
EU states, Norway, Liechtenstein, Switzerland, and Iceland.
Please be aware that each country may have different drone law rules and regulations, so ensure that you are always compliant with them.