What you want IINA to do:

The Jump To... item the Playback menu doesn't support entering only seconds.

What IINA does currently:

If I open the Jump To... dialog and type 40 and hit RETURN, it does nothing. Could even be considered a bug.
Same result when typing :40 (nothing). To go to t=40s, I need to type 0:40.

Why you think this should be added:

Would be easier & more user friendly. I don't like unnecessary typing. Especially typing colons. Using the Shift key is laborious. Sometimes would be nice to type 300 instead of 5:00 to go to t=5m.

Examples of other projects that have something similar:

YouTube's t URL param allow different time formats, for example either t=121 or t=2m1s