I can’t add any text in the label field in textbox properties.
If you expect any answer at all, edit your question to describe your goal (what are these text boxes for?), how you implement it and what goes wrong.
A common error for newbies is to request help on their implementation rather than to describe the final goal. You might have followed a wrong track by lack of knowledge of Writer capabilities.
Please do not use Add Answer but edit your original question to enhance the details of your question (answers are reserved for solutions to a problem on this Q&A site).
And mention your OS name and LO version.