How do I write into hidden paragraph

I can’t get to use conditional hidden paragraph.
I created a variable to control it, let’s say OnOff to TRUE, inside a hoidden field.
Then I want to create a field->functions->hidden paragraph.
I put condition OnOff EQ TRUE, but the Hidden Text field is grayed out and not writable.
So all I get is a useless paragraph.
I then tried to choose “View > Hidden Paragraphs” to see if If that’s the way to put something in the paragraph, but nothing changes: if I double click it, or put the cursor in front or after, there’s no way to write something in that paragraph.
By the way, in my Italian interface, the function is called “empty paragraph” wich is what it actually does, which use I still have to guess.
Any help? thank you

I also broken my head when first met with this field.

The paragraph you put the field “hidden paragraph” in, will be completely hidden, including what isn’t in field. You just put any text, then put field “hidden paragraph” to any place of the paragraph (begin, end of line, anywhere in middle). And if condition will cause paragraph to be hidden, it will be hidden as a whole, not just the field.

To hide only portion of text, there is “hidden text” field.

Also note, to actually cease to see “hidden” paragraph you must un-check (in main menu) View → Hidden paragraphs. But independently of that switch, it won’t be on page preview, neither printed, unless you check “Hidden text” in Print dialog.

Thank you LogicDaemon, I tested your soulution and it works with a variable in the text.
Strangely it doesn’t work if I try to control it through a document property.
Anyway your answer saved me.
Thank you
Here’s my test file (there are two variables: one “test” at the beginning of the file and one other “Onoff” in the document properties):
onofftest.odt

yes, it only works with variables. Not sure if document properties are accessible any similar way.
To change variables without opening field edit dialogue each time, you can use Input Field field from same (Variables) tab of Insert field dialogue. Then, if you’ll save your document as template (*.ott, it’s in dropdown lost of formats/extensions of save file dialogue), LO will request values for all such variables when you’ll open it (also, it will be treated as a new document).