Note before starting: This will require a setting in "settings -> order -> Opening purchase orders"
There is a checkbox for "Save and open purchase order" from the purchase order line creation form
If Checked "Save & open order" button appears when creating/editing a pending purchase order line
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Create Purchase Order (PO). Click "Save & close" button.
Add at least one purchase order line with Add PO line button.
Click "Save & open order"
Create PO
Add at least one purchase order line (POL) with Add PO line button.
Click "Save & close"
Edit PO Line details by selecting Edit under Actions button
Edit a detail of POL
Click "Save and open order"
POL is saved with correct details and Purchase order Workflow status is changed to Open
See T479341 for creating or adding an order line to an existing PO.