If you attach your template to a reply, I'm happy to simply put this in place for you.
On your form, you have a drop down control, correct? The user selects a supplier from it.
This control is bound to a main data source field.
If you right click the control, and select the control's properties, the main data source field's name should show in the control properties.
In the third dropdown in the screen shot you display, do you see where you have currently selected the Acct NO field?
Instead, you need to select your main data source field that is bound to the drop down where your users select a Supplier. I'm sorry, I just don't know how else to explain this.