Power Query - Add column from related table

Hi MichaelF1 ,

You can achieve it by the following methods:
1. Power Query: Add a custom column as below in Table2

= Table.AddColumn(#"Changed Type", "Cust_PostCode", each Table1[Cust_PostCode]{List.PositionOf(Table1[Cust_Ref],[Cust_Ref])})

In addition, you can refer the following blog to achieve it, there are two methods(merge method and add a custom column method) include in this blog.

VLOOKUP in Power Query Using List Functions

Merge method

Add a custom column method

2. DAX: Create a calculated column as below to get it

Column = 
CALCULATE (
    MAX ( 'Table1'[Cust_PostCode] ),
    FILTER ( 'Table1', 'Table1'[Cust_Ref] = 'Table2'[Cust_Ref] )
)

Best Regards

Hi MichaelF1 ,

You can download my sample pbix file from this link to get more details about creating custom column in Power Query Editor...

Add a custom column in Power BI Desktop

Best Regards