Firstly, tax withholding must be enabled within the invoicing settings.
In the top right corner of your main dashboard click on the name of your company and choose "Settings" from the drop-down menu.
On the left side of the screen, you can see the settings for your personal profile, and company profile. From here, click on the option for "Invoicing".
Once selected, scroll down the page until you see the option "Enable Tax withholding". Click here to activate the function, and Tax withholding will then become enabled once the button switches to blue.
Next, choose the reason of withholding from the drop-down menu. The percentage in the fields Rate and Taxable will change automatically.
Finally, to add a tax withholding to the invoice, open the invoice you need from the main dashboard. Then, in the bottom right corner, click on "Apply tax withholding".
Now your invoice is ready to be sent to your customer!