Hello:
You can do this using the concat function to concatenate your fields (along with the .xml extension) and translate to remove the dashes from the date. Select the Insert Formula button next to the text box for the name. In the Insert Formula window, select Insert Function. From the available Text functions, select concat. The first field would be your office field. After the comma, select Insert Function again, and this time select translate from the available Text functions. The translate function takes three arguments. First is the field you wish to translate (in your case, the date field). Second is the string you wish to translate (here it is the "-"). And, finally, the replacement string. We want to replace the dashes with nothing, so we use blank quotes. After another comma, add the file extension. Your finished formula will look like this:
concat(office, translate(date, "-", ""), ".xml")
And the result should be the file name format you were looking for.