I wanna know which extension support to add the default post title after I selected the tag.
jdysya there is an extension allowing you to add discussion/post templates - but not the discussion title: https://discuss.flarum.org/d/23950-discussion-and-reply-templates. Still, it might fit best to be added to this extension as a feature.
GreXXL Oh, thank you! I will ask for this feature by a issue.