I read somewhere that with correct tongue position, the area underneath your jaw (between your chin and neck) should feel hard. If it’s soft, the tongue isn’t in the right place. Is this true?
Yes, the area under the jaw should tense up and tighten. You might not be able to feel it if you have a considerable layer of fat.
The tongue is attached to your chin/mandible. IF you shove it up onto the hard palate without touching front incisors and the back of the tongue is in position, the area below your chin should be hard, because the muscles tense up (basically your tongue). See picture in the attachment.
Your tongue might not reach the hard palate if you have an inter-molar distance that is less than 35 mm, in which case one should consider expanding the arch to 40 mm via a device.
EDIT: I struggled for almost 3 months before I could have a kind of almost proper posture and I have above 40 mm inter-molar -.- if your tongue has never been up there it will feel odd.