As Spursdebs says you are too late to use the FTV route. Either just buy a return trip somewhere starting by 15 May (if you can find availability, SH is more likely than LH although New York in economy currently has a lot of availability in May) or I suggest buy a ticket to a destination you want to visit twice and book the outbound before 15 May but the return for as long ahead as you can then buy another return ticket starting in the destination for dates in between. That way you get to use the voucher and get two trips.