Excel's built-in Data Validation unfortunately doesn't offer a direct search function within dropdown lists. However, you can achieve a similar outcome using a combination of Data Validation and a helper column with a formula. Here's how:

1. Prepare your Data List:

- Create a separate table or list containing the items you want in your dropdown list (ideally on a different sheet for clarity).

2. Create a Search Helper Column (Optional):

- In a column next to your data list, enter a formula to create a case-insensitive search criteria. You can use the CONTAINS function for this purpose.
- For example, if your data list is in cells A2:A10 and the search helper column is in B2:B10, you can enter the following formula in B2 and copy it down:
`=CONTAINS(A2, LOWER(C1))` (Replace C1 with the cell where users will type their search term)

3. Set up Data Validation:

- Select the cell(s) where you want the searchable dropdown list.
- Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
- In the "Settings" tab, under "Allow," choose "List" from the dropdown menu.

4. Define the Source with a Twist:

Here's the key step: Instead of directly referencing your data list, you'll use a formula that filters the list based on the search term.
- In the "Source" box, enter the following formula (assuming your data list is in A2:A10 and the search helper column is B2:B10):
`=INDEX($A$2:$A$10, MATCH(TRUE, $B$2:$B$10, 0))`
- This formula uses INDEX and MATCH to dynamically return the filtered list based on the search criteria in the helper column (B2:B10).

5. Link the Search Term Cell:

- Replace `C1` in the formula from step 2 (search helper column) with the actual cell reference where users will type their search term.
- Now, when users type a search term in the designated cell, the helper column will evaluate each item in the data list and the Data Validation formula will pick up the filtered list based on the search criteria.

6. Additional Options (Optional):

- Check the "Ignore blank" box if you want to allow users to leave the cell empty.
- Check the "In-cell dropdown" box to display a small down arrow within the cell for easier access to the dropdown list.

7. Click OK:

Once you've completed these steps, you should have a dropdown list that updates dynamically based on the user's search term. While not a built-in search function, this approach offers a good alternative for creating a searchable dropdown list using Excel's Data Validation feature.