In the already-existing 60100 Customer Category PKT codeunit, we embed the requirements and write the test story, which leads us to the following test function:

[Test]
procedure AssignDefaultCategoryToCustomerFromCustomerCard()
// [FEATURE] Customer Category UI
var
    Customer: Record Customer;
    CustomerCategoryCode: Code[20];


begin
    // [SCENARIO #0007] Assign default category to customer from
    //                  customer card
    // [GIVEN] A non-blocked default customer category
    CustomerCategoryCode :=
        CreateNonBlockedDefaultCustomerCategory();
    // [GIVEN] A customer with customer category not equal to
    //         default customer category
    CreateCustomerWithCustomerCategoryNotEqualToDefault(Customer);
    // [WHEN] Select "Assign Default Category" action on customer
    //        card
    SelectAssignDefaultCategoryActionOnCustomerCard(
        Customer."No.");
    // [THEN] Customer has default customer category
    VerifyCustomerHasDefaultCustomerCategory(
        Customer."No.",
 CustomerCategoryCode);
end;