Does service mapping support 'if then else if then' for literal

I want to set Shipment Priority depending on source of the Transfer Order. Especially for those with source: DOS, EXT, INV and POR

When using a literal and set ShipmentPriorityLookup using an expression with if, it works. But not if then else if …

Sample that works in case source is self service procurement:

if (SupplyHeader.SupplyOrderSource == 'POR') {return 'SSP';}

But no result for any source when using:

(SupplyHeader.SupplyOrderSource == 'POR') {return 'SSP'} else if {SupplyHeader.SupplyOrderSource == 'EXT') {return 'PAR'} else {return 'OTHER'}