Inject column as parameter in a defined function inside a join

CliveWatson 

I've simplified it in order that you see what I mean.

This works (using now() in the function call:

let daysOfMonthNoWeekends=(_event_date_t:datetime) {
toscalar(range days from startofmonth(_event_date_t) to endofmonth(_event_date_t) step 1d
| where dayofweek(days) between(1d .. 5d)
| count)
};
//
MyTable_CL
| extend daysOfMonthNoWeekends = daysOfMonthNoWeekends(now())

And this doesn't works:

let daysOfMonthNoWeekends=(_event_date_t:datetime) {
toscalar(range days from startofmonth(_event_date_t) to endofmonth(_event_date_t) step 1d
| where dayofweek(days) between(1d .. 5d)
| count)
};
//
MyTable_CL
| extend daysOfMonthNoWeekends = daysOfMonthNoWeekends(TimeGenerated)
//or with another column of MyTable like event_date_t fails too
//| extend daysOfMonthNoWeekends = daysOfMonthNoWeekends(event_date_t)

Error:

Semantic error: '' has the following semantic error: Unresolved reference binding: 'TimeGenerated'.

For the record I pretend to add a column with the number of days without weekends in a month based on a column which is a date.