Expression to choose the value of this row

[Tiger1]

I need to choose the value of Current Trx Amount] from a particular row (chosen by the user). I have an expression, but it is wrong. How can i do this?

Expression:
SELECT(AR[Current Trx Amount], ([Customer Number] = [_THISROW].[Customer Number]))

Table = AR
Value needed per row = [Current Trx Amount]