Access a UDF in an xlsx file

There are a couple of ways to do this. Where is your UDF defined? If the code is in PERSONAL.xlsb, then it will always be available to your xlsx file.

You can put the code in a .xlsm file and convert it to an add-in. This requires the add-in to be installed.

Who will use the file? If you want to give your file to another user then it's a bigger problem.