how to create a column with the username () function

I want to create a create a column using the following function: ID_market = if ([ID_email] = username(); "MyCompany";"Market")

I want every user to see its market share. After creating the column above would create a measure of "market share"using the function "calculate".

But I can not create the column "ID_market"

how do i do that?!?!