Display "Multiple" values with selected value in column of matrix table

[Jessica_17]

I have a matrix table like this,

where I would want, If a user selects a single customer from a slicer,

then the first row should always be fixed and shown on top, while the selected customer should be shown on bottom of it, if the user selects multiple customers (in this scenario "AGB","CRM"), then the first row should be fixed on top as is, and in second row it should show like this

I have tried this method,

MonthSelection = 
VAR _Count = COUNTROWS(VALUES(_Date[Month]))
VAR _Concat = CONCATENATEX(VALUES(_Date[Month]),[Month],",")
RETURN IF(_Count>2,"Multiple Selection",_Concat)

but it is not taking measure in rows section of matrix table, and Customer is a column and I would require it in rows section only.

Can anyone please help me in this?

[ToddChitt]

Sorry, but your description of the issues does not make sense. 

"...the first row should be fixed on top..." What does that mean? 

[Jessica_17]

HI @ToddChitt 

Sorry for the confusion, By first row I meant, customer which is NAT should always shown at top irrespective what other customers are selected.