I'm curious if I'm able to develop a text field in inDesign that does a character count of another text box. I would like it to update automatically so I can see how many characters I have left (it's for Facebook ads)Text.
Not necessary because it's already a feature in InDesign! Open the Info panel (Window > Info), choose the Type tool, and put your cursor in a story (linked text frames). Info panel will give you a live count of characters, words, lines, and paragraphs.