Autosize text frame without losing the shape
I am quite new to InDesign, and I am currently trying to make a template document for my work. I have made a cover page, and I know that the titles will be in different lengths from time to time so I made a text frame which resizes automatically. Now the problem I came across is that the shape (the angle on the right side) of my text box changes with the size.
How do I make it keep its shape?
This is the original shape I want to keep:

This is the changed shape after the autosizing:



