First, search the help docs and this forum. Maybe your question has been answered! The debugging steps can help, too. Still stuck? Delete this line and proceed.
It looks like you accidentally posted before finishing your post. I’m going to unlist this for now.
Are you looking for an automatic solution or something where you retroactively want to add the tags or links to all tasks within a given heading?
For the retroactive method, you could use search and replace to make the process a little faster. The exact method would depend upon where you want the tag or link to be placed in the task. If you wanted them placed at the end, you could use a regex search and replace in a program like VSCode, making use of capturing groups. The search query would be - \[(.)\] (.*) and for replace you would use - [$1] $2 #mytag. Definitely use this method with caution and after having backed up your vault. It can be very risky.
Another option, is to build a Templater script to accomplish this. While I am unable to do this, someone here on the forum might be able to help you with that.
Good luck!