Q:

How to link a subtask to an existing issue in JIRA?

I'm making a subtask. When clicked, I want the subtask to go to an existing issue. How can I do this?

A:

From Creating a Sub-Task - Atlassian Documentation:
Converting a standard issue to a sub-task

Navigate to the issue you would like to convert.

Select More > Convert to Sub-Task.

In the Step 1. Select Parent Issue and Sub-Task Type screen, type or select the appropriate parent issue type and the new issue type (i.e.
a sub-task issue type). Click Next.

If the issue's current status is not an allowed status for the new issue type, the Step 2. Select New Status screen is displayed. Select
a new status and click Next.

In the Step 3. Update Fields screen you will be prompted to enter any additional fields if they are required. Otherwise, you will see the message 'All fields will be updated automatically'. Click Next.

The Step 4. Confirmation screen is displayed. If you are satisfied with the new details for the issue, click Finish.

The issue will be displayed. You will see that it is now a sub-task, that is, its parent's issue number is now displayed at the top of the screen.
Note: You will not be able to convert an issue to a sub-task if the
issue has sub-tasks of its own. You first need to convert the issue's
sub-tasks to standalone issues (see below); you can then convert them
to sub-tasks of another issue if you wish. Sub-tasks cannot be moved
directly from one issue to another — you will need to convert them to
standard issues, then to sub-tasks of their new parent issue.

One caveat: Sub-Tasks must be enabled and at least one Sub-Task issue type has to be defined.

