Not 100% sure what you are asking but believe you are saying that you are trying to include an IFrame in your post, but after you post it does not display.
If so then the reason for this is the forum is developed for communication using words. You are able to use some HTML such as bold, italic, hyperlinkls, images, etc. but anything which is not normally used within a forum and/or could be a security risk would be stripped by the security filters.
For example IFrames can not be used in posts as the IFrame could be to a web site that contains malware, Cross-Site Request Forgery (CSRF), Cross-site scripting (XSS), or aother security vulnerabilities which could be used to infect anyone viewing the post with malware, virus, hack their forum account, etc.
As it is a big security risk to allow IFrames within posts and as IFrames are not normally needed within the context of a forum post they are stripped by the security filters.