The MPARAMETER keyword defines the value of a parameter used in Power Query for a dynamic M parameter in DirectQuery connections.
The name corresponds to the parameter name defined in the Power Query Editor; if the M parameter name contains a space, it should be surrounded by single quotes.
The following example shows different types of dynamic M parameters for a single DAX query.

DEFINE 
    // Single date parameter
    MPARAMETER DateParameter = DATE ( 2022, 5, 23 )
 
    // Single string parameter, where the parameter name includes a space
    MPARAMETER 'Country Parameter' = "Canada"
 
    // Multi-select enabled on dynamic M parameter
    MPARAMETER BrandParameter =  { "Contoso", "Fabrikam" }

EVALUATE 
SUMMARIZECOLUMNS ( 
    'Date'[Date],
    "Sales", [Sales Amount]
)