To set a partial SL you need to first make sure that you have either FN1, FN2 or FN set to Partial SL in the expert advisor settings. Afterwards, pressing that button will pop up the partial SL line. After placing it at the right price, you will need to activate it with the button that appears in the bottom left corner of the chart.
You can only activate it once there is at least one trade open.
Dy default, the Partial SL il liked to the FN button and it closes 50% of the position.