You can set this to either required or not, depending if the clients wants it to be required when creating a new PO.

In order to set this up to go Admin > PO Components

Click on 'Add Section' on the Component Structure box:

This will then add a new section like shown below:

Click onto the pencil icon next to the new section and this will then present you with the following box:

Here you can enter the Title IE: ACH Transfer Required and the description IE: If an ACH transfer is required, please enter the following details.

You will want to set this as visible in order to see this when creating a P.O

You will then need to create the component so click on the pencil icon next to 'New Component' when hovering over the pencil icon it will say 'Edit Component' 

Here you can then enter the Label IE ACH Account Number, I would also suggest changing the PDF Position so that all the text lines up when previewing this and check to make it visible and appear in the preview.

You will then need to follow the same steps above and create a new component for adding in the ACH Routing Number like shown below:

Then use the preview icon before clicking save an next so you can see how it will appear: