Increase shadows and contrast but keep developing time standard with TMAX 400 Film

You cannot change the contrast of the negative without changing the amount of development.

Further, the amount of contrast control available for T-Max films is limited -- only the P3200 is made to accommodate significant changes in contrast ("push") to simulate a 3200 ISO speed (the native speed of that film is about 1000). T-Max 400 has a relatively limited amount of "push" available.

In general, however, to lose shadow detail, you would expose less. This is commonly combined with developing more than standard (which you can request at the lab, if you're not processing the film yourself) to give mid-tones similar to a correctly exposed negative. If you just underexpose and then give normal development, you'll get negatives with lower than normal density; depending on your scanner you may or may not be able to bring the tones back to your usual mid-tone range.

This will give the harder contrast you request, at the scanning stage -- but to get this in the negative, you would need to give extended development. Most labs charge extra for this, because they can't batch your film in with other rolls of the same film type, but instead must handle it more individually -- but anywhere that processes B&W film should be able to give a one stop push. You just have to request it (and you may have to ask them how to request a push process).

You would then set your meter to 800, or for auto exposure cameras dial in "-1" exposure adjustment, shoot normally, and process with the "Push +1" option.