I want to know how to save the reference of a transform on a custom inspector

I have made a custom timeline track , and I need to set and save a tranform reference on my custom inspector.
At first, I cast the property “target” of the editor as my custom playable asset , and set the reference of the transform directly. Like this:
7533041--930119--upload_2021-9-29_22-14-49.png
But when I close and reopen the scene, I find the tranform reference is missing.
7533041--930107--upload_2021-9-29_22-5-22.png
Then I found the class ExposedReference may can resolve my problem. I try to use it like this:
7533041--930125--upload_2021-9-29_22-17-8.png
And I tried 3 kind of methods to set the reference value, but no one worked well.


So I wonder how to make it and if have an example is better, THX for any help!!!

7533041--930119--upload_2021-9-29_22-14-49.png

Hope anyone can help me!!! Thank you!!!