I'm building a new application and I want to use the new REST api. My application will generate the contract and send to the customer to sign it.
This is something I already do using the SOAP api in another application.
Reading your documentation I saw that the only way to get the Access Token is redirecting the customer to the authentication page, but I don't want to do it because I don't want to send the documents on my customers' behalf.
How can I get the Access Token for my application be able to send the contract on its behalf? Can I do it?
It is possible to set up an access token that has the ability to send documents on behalf of any user in your account.
Set up a new application in your Adobe Sign dashboard (Account -> Adobe Sign API -> API Applications) and go through the oAuth process (A couple helpful resources: Adobe Sign oAuth Documentation, Documentation: Get the Access Token).
You'll likely want to use an authorization scope similar to the following:
After you've gone through the process once, you'll be able to use the refresh token in future requests, allowing you to send documents on your application's behalf without having to redirect users to the authentication page.