Step:1

Create parameter with string data type and select all.

Untitled.png

 

Step:2

Create Calculation [Wildcard Search]

CONTAINS("," + [Customer Parameter] + ",", "," + str(left([Customer Name],1)) + ",")

 

Step:3

Drag Customer name into row

 

Step:4

Apply [Wildcard Search] in filter and select true.

 

Step:5

Show Parameter control and type A,B,C and get names.