You can set a default value for each field on your webform. The default value you would use would be

%get[customer_name]

The text between [] is the key from your url. You can use as many as you like. So to get a first and last name you could do something like

URL:

http://mysite.com/myform?first_name=John&last_name=Doe

First Name Default Value:

%get[first_name]

Last Name Default Value:

%get[last_name]

You can set a default value for each field on your webform. The default value you would use would be

For Webform v3 and under:

%get[customer_name]

For Webform v4+:

[current-page:query:customer_name]

To get a first and last name you could do something like:

URL:

http://mysite.com/myform?first_name=John&last_name=Doe

First Name Default Value:

%get[first_name]                 //webform <= v3
[current-page:query:first_name]  //webform >= v4

Last Name Default Value:

%get[last_name]                 //webform <= v3
[current-page:query:last_name]  //webform >= v4