diff --git a/changelog/19429.txt b/changelog/19429.txt
new file mode 100644
index 000000000..341fbf5a7
--- /dev/null
+++ b/changelog/19429.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+ui: pass encodeBase64 param to HMAC transit-key-actions.
+```
diff --git a/ui/app/templates/components/transit-key-action/hmac.hbs b/ui/app/templates/components/transit-key-action/hmac.hbs
index e739201e9..5c45924c5 100644
--- a/ui/app/templates/components/transit-key-action/hmac.hbs
+++ b/ui/app/templates/components/transit-key-action/hmac.hbs
@@ -1,4 +1,9 @@
-<form onsubmit={{action @doSubmit (hash input=@input algorithm=@algorithm key_version=@key_version)}}>
+<form
+  onsubmit={{action
+    @doSubmit
+    (hash input=@input algorithm=@algorithm key_version=@key_version encodedBase64=@encodedBase64)
+  }}
+>
   <div class="box is-sideless is-fullwidth is-marginless">
     <NamespaceReminder @mode="perform" @noun="HMAC creation" />
     <div class="content">
diff --git a/ui/tests/integration/components/transit-key-actions-test.js b/ui/tests/integration/components/transit-key-actions-test.js
index 3d6c5ca47..6a3926e2d 100644
--- a/ui/tests/integration/components/transit-key-actions-test.js
+++ b/ui/tests/integration/components/transit-key-actions-test.js
@@ -322,6 +322,8 @@ module('Integration | Component | transit key actions', function (hooks) {
     `);
     await fillIn('#algorithm', 'sha2-384');
     await blur('#algorithm');
+    await fillIn('[data-test-component="code-mirror-modifier"] textarea', 'plaintext');
+    await click('input[data-test-transit-input="encodedBase64"]');
     await click('button[type="submit"]');
     assert.deepEqual(
       this.storeService.callArgs,
@@ -331,6 +333,7 @@ module('Integration | Component | transit key actions', function (hooks) {
         id: 'akey',
         payload: {
           algorithm: 'sha2-384',
+          input: 'plaintext',
         },
       },
       'passes expected args to the adapter'