I am using docusign-esign node SDK and i want to change the Sender's name(as of now docusign using the Name associated with the account)

 const accessToken = await getAccessToken();
    const apiClient = new docusign.ApiClient();
    apiClient.setBasePath(`${process.env.DOCUSIGN_RESTAPI_BASE_URL}/restapi`);
    apiClient.addDefaultHeader("Authorization", "Bearer " + accessToken);
    docusign.Configuration.default.setDefaultApiClient(apiClient);
    const envelopesApi = new docusign.EnvelopesApi();
    const pdfBuffer = await fetchPdfBuffer(pdfUrl);
    const envelopeDefinition = await buildEnvelopeDefinition(parties, pdfBuffer);
    const results = await envelopesApi.createEnvelope("7578669", {
      envelopeDefinition
    });

As I am using envelope definition and there is a field with name sender in envelope object definition (<https://developers.docusign.com/esign-rest-api/reference/Envelopes/Envelopes/create>
How can I use this sender field to change the sender of the envelope?

And how can we use Send-On-Behalf functionality using deocusign-esign node sdk?

When you make API calls to create an envelope, the sender is the authenticated user that was used to obtain the access token.
That means that you cannot change it without getting a new access token for a different user.
To do that, unless you want that other user to interactively authenticate to DocuSign every time, you will need to use JWT authentication.
With JWT authentication, when you obtain the access token, you specify the userId for the user you want to act on behalf of.
That user must give your app consent once to allow it to use the API of behalf of this user.
To learn more, read this:
https://developers.docusign.com/esign-rest-api/guides/authentication/oauth2-jsonwebtoken

Note that your application can create different access tokens (using the JWT Grant flow) as needed to send as user 1, user 2, etc.

DocuSign's OAuth implementation does not support the old "Send on behalf of" any more. Instead, using OAuth, your app obtains a token to impersonate someone and can then send an envelope for them.

However, there is a limit to the number of current access tokens for one user that you can create, so you should cache them until they expire.

For example, you have two DocuSign account users (both can send envelopes), Sam and Samantha:

1. App uses JWT Grant to get an access token that impersonates Sam
2. App uses the token to send an envelope "from" Sam
3. App caches Sam's token. (Record the token's expiration time.)
4. App uses JWT Grant to get an access token that impersonates Samantha
5. App uses the token to send an envelope "from" Samantha
6. App caches Samantha's token. (Record the token's expiration time.)
7. App wants to send another envelope "from" Sam. It uses the cached token. If the token has expired then use JWT Grant to get a new one.