Slicer on measures

viswaaa , is the direct lake/Live semantic model, then you need to create it at the source. If this direct query or Import mode, you should be able to create a new column. 
You can create a measure but that can not be used in Group by 

Search = CountX('Fabric Items', 'Fabric Items'[Item name]& " - " & related('Workspace'[workspacesname])) 

Assuming Workspace is dimension 

Hi viswaaa , you can try these steps to solve your query 

Step 1: Create a new table inside your report (allowed even on semantic model)

Go to Modeling → New Table and create:

DAX 
Search Table =
DISTINCT (
    SELECTCOLUMNS(
        'Fabric Items',
        "SearchText", 'Fabric Items'[Item Name] & " - " & RELATED('Workspaces'[WorkspaceName])
    )
)

If RELATED does not work because of inactive relationships, use this:

DAX
Search Table =
DISTINCT (
    NATURALLEFTOUTERJOIN (
        SELECTCOLUMNS('Fabric Items',
            "ItemName", 'Fabric Items'[Item Name],
            "WorkspaceID", 'Fabric Items'[Workspace ID]
        ),
        SELECTCOLUMNS('Workspaces',
            "WorkspaceID", 'Workspaces'[WorkspacesID],
            "WorkspaceName", 'Workspaces'[WorkspaceName]
        )
    )
)

Then create:

DAX
Search Table = 
ADDCOLUMNS(
    Search Table,
    "SearchText", [ItemName] & " - " & [WorkspaceName]
)

Step 2: Use SearchText as SLICER

Step 3: Filter your visuals using Relationships or TREATAS
If the search table has no relationship, use:

Search Filter =
TREATAS(
    VALUES('Search Table'[ItemName]),
    'Fabric Items'[Item Name]
)

Add this measure to the visual-level filter (set to not blank).

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I'm guessing you're trying to get the data from the Fabric Capacity Metrics semantic model?

At our company we extracted the data that we're using from the semantic model using a notebook and Python. It does require some work, but then you can build your own report with selected data from the Metrics report.

I don't have a full documented step-by-step guide on how we dit this, but I did found this blog detailling how you can set this up:

How to extract data from the Fabric Metrics App – Part 2 – PBI Guy

Extra information

https://learn.microsoft.com/en-us/fabric/data-science/read-write-power-bi-python

Hi viswaaa ,

Thanks for reaching out to the Fabric Community.
Since your report is connected to a read only semantic model, Power BI won't allow creating calculated columns or tables directly in the report. Because of this, a combined field like Item Name Workspace Name cannot be created at the report level. This is expected behaviour, as any structural changes have to be done in the main dataset itself.

For now, the simplest and most practical method is to use two slicers, one for Item Name and one for Workspace Name and turn on the search option in both. This will let users quickly filter and narrow down results on both fields. If there’s a relationship between Workspaces and Items in your model, you can also use a hierarchy slicer so users can drill down from Workspace to Item in a single slicer. For a long term and proper solution that gives you one combined searchable field, the dataset owner or admin will need to create a concatenated column in the source model and republish the dataset. Once that is done, the combined Search field will appear directly in the report and can be used in a slicer hopefully.