DAX for Case statement

Here is a working example that (I think) solves your problem.

Kudos is appreciated if the solution was useful  😀

Code for reproducing the tables used in the solution is included.

Steps for creating a summarized table that implements the desired logic for summarizing activites from 2 different tables in Power BI:
0) Genereate reproducible data:
Table Warehouse Activity

WarehouseActivity = 
    UNION(
        SELECTCOLUMNS(
            ROW( "WarehouseCodeKey", 1, "WarehouseCode", "A", "WarehouseActivity", 10 ),
            "WarehouseCodeKey", [WarehouseCodeKey],
            "WarehouseCode", [WarehouseCode],
            "WarehouseActivity", [WarehouseActivity]
        ),
        SELECTCOLUMNS(
            ROW( "WarehouseCodeKey", 2, "WarehouseCode", "B", "WarehouseActivity", 20 ),
            "WarehouseCodeKey", [WarehouseCodeKey],
            "WarehouseCode", [WarehouseCode],
            "WarehouseActivity", [WarehouseActivity]
        ),
        SELECTCOLUMNS(
            ROW( "WarehouseCodeKey", 3, "WarehouseCode", "---", "WarehouseActivity", 30 ),
            "WarehouseCodeKey", [WarehouseCodeKey],
            "WarehouseCode", [WarehouseCode],
            "WarehouseActivity", [WarehouseActivity]
        )
    )

Table WareHouseSiteCode

WarehousSiteCode = 
    SELECTCOLUMNS(
        ROW( "WarehouseCodeKey", 3, "WarehouseCode", "---", "WarehouseActivity", 40 ),
        "WarehouseCodeKey", [WarehouseCodeKey],
        "WarehouseCode", [WarehouseCode],
        "WarehouseActivity", [WarehouseActivity]
    )

1) In Power BI model view:
Connect your tables
- WareHouseActivity and
- WareHouseSiteCode
using warehouseCodeKey

2) Add column WarehouseActivityAdjused in table WareHouseActivity based on the sum-logic you described:

WareHouseActivityAdjusted = IF(
                [WarehouseCode] = "---",
                LOOKUPVALUE(
                    WarehousSiteCode[WarehouseActivity],
                    WarehousSiteCode[WarehouseCode], "---"
                ),
                [WarehouseActivity]
            )

 , as shown in snip below:

3) Add a summary table that summarizes the data by WareHouseCode

WareHouseActivity summarized = SUMMARIZE(
    WarehouseActivity,
    WarehouseActivity[WarehouseCode],
    "
WareHouseActivityAdjusted sum
", SUM(WarehouseActivity[WareHouseActivityAdjusted])
)

4) Connect summarized table to datamodel for filtering:
In order to filter the summarized table using your first table, you can connect them as shown below: 

🤺Result:


First table can be used as slicer /filter:

An equal solution using a simple SQL join and CASE statement is included here:

# 0) Generate test data
WITH warehouseActivity AS(
  SELECT 
    1 AS WarehouseCodeKey,
    "A" AS WarehouseCode,
    10 AS WarehouseActivity
  UNION ALL
  SELECT 
    2 AS WarehouseCodeKey,
    "B" AS WarehouseCode,
     20 AS WarehouseActivity
  UNION ALL
  SELECT 
    3 AS WarehouseCodeKey,
    "---" AS WarehouseCode,
    30 AS WarehouseActivity
),

WarehousSiteCode AS(
    SELECT 
    3 AS WarehouseCodeKey,
    "---" AS WarehouseCode,
    40 AS WarehouseActivity
)

# SUM WareHouseActivity by WarehouseCode.
# If WareHouseCode meet condition, use value from table WarehousSiteCode
SELECT 
  a.WarehouseCode,
  SUM(
    CASE WHEN a.WarehouseCode="---" THEN b.WarehouseActivity
    ELSE a.WareHouseActivity
    END
  ) AS WarehouseActivity FROM 
  warehouseActivity a  
  LEFT JOIN WarehousSiteCode  b
  ON a.WarehouseCodeKey=b.WarehouseCodeKey 
  GROUP BY WarehouseCode

JenniferZen 
Please Accept as Solution if resolved your problem.