I think you can use any format as long as you can make some good changes in it. Using of the same message in a thread might be disliked by the creator of the message and may lead to plagiarism and that's why it's always recommended to learn from someone's ideas and then make your own improved ideas from the learnt knowledge.

As long as your signature renting advertisement is unique and it doesn't contain the exact text from someone else's advertisement, you can post it without worrying about anything. I think reading someone's ideas and then making better ideas yourself for your advertisement doesn't fall into the domain of plagiarism.

We aren't perfect as humans and we learn things from each other, and that's why it's better to learn from those who have been doing good, and then come up with your own thing. We learn from books, from forums, from general texts, and from many other things and when we represent the learnt things in our own writing style then that doesn't fall into the domain of plagiarism.