Adds an entry for targetting preview branches

Author: samejr

docs/management/authentication.mdx

@@ -94,4 +94,44 @@ await envvars.upload("proj_1234", "dev", {
   },
   override: true,
 });
+```
+
+### Preview branch targeting
+
+When working with preview branches, you may need to target a specific branch when making API calls. This is particularly useful for managing environment variables or other resources that are scoped to individual preview branches.
+
+To target a specific preview branch, include the `x-trigger-branch` header in your API requests with the branch name as the value:
+
+```bash
+curl --request PUT \
+  --url https://api.trigger.dev/api/v1/projects/{projectRef}/envvars/preview/DATABASE_URL \
+  --header 'Authorization: Bearer <token>' \
+  --header 'x-trigger-branch: feature-xyz' \
+  --header 'Content-Type: application/json' \
+  --data '{
+    "value": "your_preview_database_url"
+  }'
+```
+
+This will set the `DATABASE_URL` environment variable specifically for the `feature-xyz` preview branch.
+
+<Note>
+  The `x-trigger-branch` header is only relevant when working with the `preview` environment (`{env}` parameter set to `preview`). It has no effect when working with `dev`, `staging`, or `prod` environments.
+</Note>
+
+#### SDK usage with preview branches
+
+When using the SDK to manage preview branch environment variables, the branch targeting is handled automatically when you're running in a preview environment with the `TRIGGER_PREVIEW_BRANCH` environment variable set. However, you can also specify the branch explicitly:
+
+```ts
+import { configure, envvars } from "@trigger.dev/sdk";
+
+configure({
+  secretKey: process.env["TRIGGER_ACCESS_TOKEN"], // starts with tr_pat_
+  previewBranch: "feature-xyz", // Optional: specify the branch
+});
+
+await envvars.update("proj_1234", "preview", "DATABASE_URL", {
+  value: "your_preview_database_url",
+});
 ```