You can add a client action for your clients to pay their taxes directly on the IRS website.
You can either include the client action as part of your tax template OR create a client action for a specific client on their tasks tab.
Create a title and due date, then add the IRS payment link: https://www.irs.gov/payments
The client will find this action item in the portal under the client actions. Once they click on view from the client action to pay their taxes directly to the IRS they will be taken to the IRS payment webset.