Measure by comparing two columns in two different table

This solution only requires Tables 1 and 2.  No bridge table, no relationships.

Add this calculated column to Table 1 - Spend:

Preferred =
VAR Test =
    COUNTROWS (
        FILTER (
            PreferredSupplier,
            PreferredSupplier[Supplier] = Spend[Supplier]
                && PreferredSupplier[Category] = Spend[Category]
        )
    )
RETURN
    IF ( Test = 1, "Yes", "No" )

The Test variable checks to see if there is a match against the Preferred Supplier table, matching both on Supplier and Category.  If there is, 1 row will be returned.   Then the result in the column will be "Yes".

Then you can create the following measures:

[Total Spend] =
SUM ( Spend[Spend] )

[Approved Spend] =
CALCULATE ( [Total Spend], Spend[Preferred] = "Yes" )

[% Approved Spend] =
DIVIDE ( [Approved Spend], [Total Spend] )

Hi ChrisHaas

Your solution is working fine with small correction but I dont know why

Preferred =
VAR Test =
    COUNTROWS (
        FILTER (
            PreferredSupplier,
            PreferredSupplier[Supplier] = Spend[Supplier]
                && PreferredSupplier[Category] = Spend[Category]
        )
    )
RETURN
    IF ( Test = 1, "Yes", "No" )

In the above calculated column I chnaged the If statement as

 IF ( Test = 0, "No", "Yes" )

remining everything I keep as you mentioned and working perfect.

But I said I still dont know what and why it gave wrong value first and right value once I changed the if statement

Thanks for the solution, it works now