Using an IF statement inside the sourcexpr of a textbox on a Form

I need to ajust the value inside a textbox according to a record value.

I cannot use a function stored in the form because I cannot edit them (since I'm an end user without developper license).

According to Mircrosoft : A source expression is a C/AL expression that defines where to find the value of a control.[...]
A source expression can be as simple as the name of a table field; it can also be a complex C/AL expression.

I'm trying to do something like : =IF("Quantity">0,'The quantity is '+Quantity','You must specify a quantity')
Is there a trick which could let me write a "single line IF statement" ?

Thanks in advance.