This seems to work, although I haven't tried it with a file that's hidden by the FTP server as I don't have anything to test with. It works with files that aren't hidden by the server, at least:
<?xml version="1.0"?>
<button backcol="none" display="both" textcol="none">
<label>File Copy Prompt</label>
<tip>Asks you for the name of a file to copy from the current directory.</tip>
<icon1>#copy</icon1>
<function type="normal">
<instruction>Copy FILE "{dlgstring|Enter name of file to copy}"</instruction>
</function>
</button>
See here if you don't know what to do with the above code block.