At the bottom of the New Customer Invoice data entry form is a field with the Tax Account information. Click the pulldown triangle next to *Sales Tax*. Click <New> in the popup menu. Enter the name of an existing account, enter the required tax rate. From now on this will be the account used to record the amount of sales tax collected with every new invoice. (Verify the correct account is used when you write your next new invoice)
I don't think you should (or need to) use savings goals for the purpose of withholding Sales tax.Just write a check every month or quarter as required in the amount of the *Sales Tax* balance from your checking account and categorize it as a transfer from [*Sales Tax*]
In some versions of Quicken the *Sales Tax* account is/was created as a hidden account.Use Tools / Manage Hidden Accounts to remove the "Hide ..." flags from the account so it appears on the Account Bar.