I think this is a dupe of T146843 ?

@Paladox is there another way to add a parent task?

@Capt_Swing in the sidebar in the task, there should be a Edit Related Tasks button, then click Edit Parent Tasks button and that should work.

@Paladox that's what I was doing :/

I also tried going to the parent task, clicking "edit subtasks", and searching for the subtask by title. But that didn't show up either.

Eventually, I successfully joined the tasks by going to the parent task, clicking "edit subtasks", filtering by "tasks assigned to me", and then scrolling down the list and finding the T148090.

Oh, did this work for you before?

In T148093#2714747, @Capt_Swing wrote:

is there another way to add a parent task?

1. Go to T148090
2. Click "Edit Related Tasks > Edit Parent Tasks"
3. Enter "T143003" in the search field
4. Get results listed within less than a second (here at least)
5. Click "Select" for the first result
6. Click "Save Parent Tasks"

@Paladox the last strategy I tried worked for me!

Yes per @Aklapper I doint think you can search for task only by there task id in the Edit Parent Tasks, you can search in the search bar though.

In T148093#2714879, @Paladox wrote:

Yes per @Aklapper I doint think you can search for task only by there task id in the Edit Parent Tasks, you can search in the search bar though.

Sorry but I don't know what that means. (And I think my steps above were pretty clear.)
Anyway, not important I guess; the search problem in this task is a valid one.