To add a residence tax, click on your Property settings and navigate to Services tab. Click on Add new service button and the following window will pop up:

Enter the service name (ie. Residence tax), add the price of the fee and select standard quantity. For residence tax this is usually per person per night, but you can set it up according to your country regulations.

Later, when invoicing your guests, simply add Residence tax as an additional item in the invoice and the total amount will be automatically added to the invoice.