How can I pre-populate a webform field with a value passed in via the link (URL)?

You can pre-populate a field in a web form based on the link used to access that form.

For example, if you provide a link such as:

https://mygroup.ourpowerbase.net/myform?civicrm_1_contact_1_contact_first_name=Joe

Then, the first name field in the web form will be populated by "Joe".

To use this option:

Configure the webform to accept values via the URL

Click Settings -> Form and scroll down to the Pre-populate section:

Ensure the "Allow all elements to be populated using query string parameters" is checked

Find the webform key for the element you want to set:

Click Build and see the key name in the Key column: