Suppose you need to generate fiscal year like 2021/22, then use this formula in a custom column where 3 means FY finishes in March month. Similarly say your financial year finishes in Jun month, replace 3 with 6..If it finishes in Jul, replace 3 with 7.
You will need to replace [Year] and [Month] fields appropriately by calendar year and calendar month
Text.From([Year]-Number.From([Month]<=3))&"/" & Text.End(Text.From([Year]+Number.From([Month]>3)),2)