How to create a link from node view and pass information to a webform
I don't think webform link field can be used to accomplish this as the purpose of Link custom attributes (YAML) is for HTML TAG attributes like 

target: _blank


You can use field_token_value to create a field on the node that has tokens as input. Install and enable the module and then add a token field.



The value of this field would be an HTML link with query parameters added (nodetitle in this example) you can browser for all available tokens under field value.



Then on the webform side you have to enable query parameter to populate fields by going to the settings of the webform &gt; form &gt; form behaviours &gt; prepopulate &gt; Allow all elements to be populated using query string parameters.



And then on the field to display the query parameter include the appropriate token.  Here adding the nodetitle to the subject field default value.



For token field to be populated a node needs to be saved.