Notebook(Alerts): PagerDuty Client URL placeholder could be default input value

## About the bug

**Steps to reproduce:**
List the minimal actions needed to reproduce the behavior.

1. Setup a Pager Duty account and generate a routing key.
2. Go to Influxdbv2 UI (note that during this test the feature flag 'notebooksNewEndpoints' was set)
3. Add test data to a bucket
4. Create an Alert Notebook
5. Use the test data to define the query
6. In the alert section choose PagerDuty

**Expected behavior:**

If a default value is useful and if it is long, which would require a lot of typing, perhaps it should not be a placeholder value but a default value for the input.

**Actual behavior:**

Firstly the sense of the label "Client URL" is not immediately clear. It will eventual generate a link to the source of the incident in Pager Duty, which in this case is a useful link back to the Notebook.
Secondly, on the first key stroke in the input, the placeholder value disappears.
It would perhaps be more useful to have this URL back to the notebook as a default input value, instead of a placeholder.

**Visual Proof:**


## About your environment

**Environment info:**

Testing in K8SIDPE Remocal

latest commit project ui

```
commit 7f6a7997396b4cbae41cb300cafaac4306e73b56 (HEAD -> master, upstream/master, origin/master, origin/HEAD)
Author: wiedld
Date: Fri Feb 18 16:40:49 2022 -0800
```

latest commit project K8SIDPE

```
commit 39c95a0c4ff722649d49c14bef5eb5f5726afebf (HEAD -> master, origin/master, origin/alchemy/automatic, origin/HEAD)
Author: Hercules Mango Churchill
Date: Sat Feb 19 01:41:48 2022 +0000
```