In the following sentence I have some problem using prepositions related to
Please click an empty line in the textbox
From the sentence above I want to say the user to (click/click on/click at) an (empty/blank) line (in/of) the textbox. The alternative words are separated with an slash. Which of them are proper?
The text box is a box on a form where you can insert some data. It can have multiple lines and some lines are already filled with text, but some lines for example like the line below are blank.
I would also be thankful to receive general guides about the ambiguities I mentioned.