According to Dana Terrace herself, Disney replies via snail mail, aka actual letters. So here are the addresses:

Disney TVA
811 Sonora Ave
Glendale, CA, USA
91201

Walt Disney Television Corporation
500 South Buena Vista Street
FGW #3052
Burbank, CA
91521

And here's the message:

Disney and associates,

I'm sorry to tell you, but you are ignoring a serious opportunity by not promoting better most of your animations. In this case, Dana Terrace's The Owl House, which is not only a mark in representation, but also has many endearing, complex and amazing characters that COULD and SHOULD be merchandise base.

For example, King, one of the main characters, is beloved by many fans, and they'd sure appreciate plushies of him.

Or maybe they'd like to have posters like the ones of Eda from season 1? Palisman plushies of all types? In-universe books about Hexside or Azura? 

You took very long to acknolowdge and allow Alex Hirsch to make Gravity Falls merch, so please, don't make the same mistake. You have made enough money with movies and streaming for all audiences. Stop to hear what your fans are asking for, embrace this opportunity to make both sides happy, please, give us official TOH merchandise.

Sincerely, 

TOH Fandom