You can call it three ways - with parameters, with DEFAULT and via EXECUTE
SET NOCOUNT ON;
DECLARE
@Table SYSNAME = 'YourTable',
@Schema SYSNAME = 'dbo',
@Rows INT;
SELECT dbo.TableRowCount( @Table, @Schema )
SELECT dbo.TableRowCount( @Table, DEFAULT )
EXECUTE @Rows = dbo.TableRowCount @Table
SELECT @Rows