Is there a way to tie a custom field to a single form to know source?

Yes, you can do this. Implementation depends on what you are trying to accomplish and what data needs to be stored with the form and how it should be used.

If all you want to do is save a custom field value as a form field value on your form, you can do this using a Hidden Field and the Default Value option under the Advanced Tab when editing the Hidden Field.

In the default value use this merge code:

{custom_field:NAME}

Replace the name with the name/key of the custom field you want to populate the default value with.

You aren't populating the field right. Adding "ModelName="Ali"" to the shortcode isn't going to populate {custom_field:ModelName}. The {custom_field:name} merge code is populated via the Custom Fields of a Post. So you would add a Custom Field to the page/post with the name of ModelName and the value of the name of the model.

See this screenshot: http://grab.by/acbS

Please note custom field names are case sensitive. So if you name it ModelName you must use ModelName and not modelname. It's best to use all lower case custom field names so you don't forget.

Custom Fields are built in WordPress functionality. They are added when creating a Post in WordPress. There is an area when creating or editing a post for custom fields. It looks like this: http://grab.by/acbS

If you don't see it when creating or editing a post, it's because it's not turned on in your screen options. Edit or create a New Post and choose "Screen Options" from the top right of the editor and check the "Custom Fields" option. See this screenshot: http://grab.by/acdV