If your purchase order is in an editable status (PR, RE, R), you can add coding lines in three ways:
Tabbing through the last field
By tabbing through each coding field, once you get to the end of the row, tap the tab button once more and the system will automatically add a new row for you underneath the preceding row.
Using the '+Add Line Item' button
You can click the '+Add Line Item' button to add as many rows as needed below the initial coding line.

Using the Edit button to add or remove lines
Selecting a row so it is highlighted in red and then choosing 'Add Above Selected' or 'Add Below Selected' to add a blank row above or below the selected row. Additionally, you can also remove lines by clicking the 'Delete' option in the Edit menu options.