How to save trim selection in iMovie?

Could you provide more details as to what you are trying to do? What do you mean by "save trim selection"?

During the editing process, iMovie automatically saves your edits as you make them. So, if you close out of a project by clicking the Project's back button at the top left of the screen, or if you close out of iMovie entirely, all your edits will automatically be saved, including any trimming that you did. When you re-open the project all of your edits will be there, including your trims.

As general information, only videos, not still images, can be trimmed. Trimming is done by placing the cursor on the left or right edge of a video clip and then press-dragging toward the middle of the clip, either left or right. You can adjust or reverse a trim by dragging back out. In other words, the trimmed out portion doesn't disappear, it just becomes invisible, so you can restore it if you change your mind. When you share out (export) the movie the trimmed out portion becomes permanent.

-- Rich