Please refer to the link on how to add a custom field on solr and implement Search Indexing Handler and add it to the content manager configuration.
In your case
<field name="Link_URL_dyn_s_str" type="text" indexed="true" stored="false"/>
Not sure what is your requirement to add the field Link_URL to solr, you can save the search folder mentioned in the document and get the records
If you want to search index to have the data of new field, you need to remove the existing collection and create a new collection , example
stop the search indexer and search host service , go to the tridion install folder / Search/solr-data/tridion ( rename the folder) and start the service again to generate new search collection and try to run the search folder again on CMS to view the results.