Basically you are asking for a WYSIWYG editor - phpBB does not have one despite many attempts over the years.
There are a couple in development if you look in the Extensions in Development forum.
David Remember: You only know what you know and - you don't know what you don't know! My CDB Contributions | How to install an extension I will not be accepting translations for any of my extensions in Github - please post any translations in the appropriate topic. No support requests via PM or email as they will be ignored