Creating a calculated field referencing a TextArea custom field

Hi All,

I need to create a calculated field on an opp which is along the lines of:

ifelse(contains($custom_text_area_field, "This text"), "It contains this text", "It doesn't contain this text")

However, I can't find the TextArea field name in the list of available fields for the formula...! Is this not possible? I can't find any reference to it anywhere online.

Many thanks,