how do i add issue schema to subtask field

[Manny Javaid]

I click on create a subtask and i click in the subtask drop down field, i see no issues. what i want to be able to select a task testing task etc that are currently in my other project 

how do i add my schema to my subtask as jira admin

[Nic Brough -Adaptavist-]

I am afraid the question is unclear.

There is nothing I could describe as an "issue schema", and there is no such thing as a "subtask field", so the initial question needs to be explained to me.

When you click on "create subtask", you should get the "create sub task" screen, which is for creating a subtask - it will be asking you to fill in some fields.  I don't know what you mean by "subtask drop down field", unless someone has added a field called "subtask".  If that is what you are asking about, you'll need to talk to the admin who created the field and ask them what it is for and why it is not doing what you expect.