There may be a simpler way, but you could try
=IF(OR(MONTH(A1)<=3,AND(MONTH(A1)=4,DAY(A1)<=5)),YEAR(A1)-1,YEAR(A1))&"-"&TEXT(IF(OR(MONTH(A1)<=3,AND(MONTH(A1)=4,DAY(A1)<=5)),A1,DATE(YEAR(A1)+1,MONTH(A1),DAY(A1))),"yy")
This assumes that the financial year boundary is between 5th and 6th of
April; modify the formula accordingly if your financial year runs
differently.