Please read this article first. Then see the steps below.
Step 1: add your Trello address* in Braintoss. Go to settings and then click on “add e-mail address”. Paste your Trello address here.
Step 2: if this Trello address is your default address in Braintoss, it will automatically send to this Trello address. If you have multiple email addresses in Braintoss you can select the email address by holding the “+” button before sending.
*You can find your Trello email address in each Trello board. Open Trello, go to a board, go to menu, go to more and there you will see the email address. Only via online and desktop versions, not in the Trello app.
For more information: https://blog.trello.com/how-to-create-trello-cards-from-email