I tried looking this up, but I didn’t see anything in the Wiki. Are these fields exposed to Picard? I want to grab the VGMDB, Discogs, and RYM URLs and set them as tags.
$if($in($lower(%url%),vgmdb),$set(VGMDB ALBUM URL,%url%))
Something like that (if that works).
The basic answer comes down to two questions:
If the answer to either of these questions is “no”, then you will need a plugin, either to process the existing response or to make additional web calls and process those responses.
A quick test on Abba Gold using the API call Picard makes with Release (and Track) relationships shows that the Release URLs are provided by MB:
<relation-list target-type="url">
<relation type="amazon asin" type-id="4f2e710d-166c-480c-a293-2e2c8d658d87"
<target id="6704aa7f-642a-4e26-836a-d14e740dd365">https://www.amazon.de/gp/product/B000001DZO</target>
<direction>forward</direction>
</relation>
<relation type="discography entry" type-id="823656dd-0309-4247-b282-b92d287d59c5">
<target id="85d4ca36-f098-4bb1-b167-bc02b508f550">http://www.abba4therecord.com/detail.php?recordID=173</target>
<direction>forward</direction>
</relation>
<relation type="discogs" type-id="4a78823c-1c53-4176-a5f3-58026c76f2bc">
<target id="9e2e3931-3955-4443-9db5-369fcd377e48">https://www.discogs.com/release/2780895</target>
<direction>forward</direction>
</relation>
</relation-list>
as well as Release Group URLs:
<relation-list target-type="url">
<relation type="allmusic" type-id="a50a1d20-2b20-4d2c-9a29-eb771dd78386">
<target id="7c8a1829-894b-433c-adc7-c9450a0d8ad8">https://www.allmusic.com/album/mw0000206799</target>
<direction>forward</direction>
</relation>
<relation type="discogs" type-id="99e550f3-5ab4-3110-b5b9-fe01d970b126">
<target id="85c2aee0-f5fc-4b7a-aee0-41503bb0b00a">https://www.discogs.com/master/12249</target>
<direction>forward</direction>
</relation>
<relation type="lyrics" type-id="156344d3-da8b-40c6-8b10-7b1c22727124">
<target id="fdd8206e-427b-4bd5-9cb1-e98fbc59aaf0">https://genius.com/albums/Abba/Abba-gold</target>
<direction>forward</direction>
</relation>
<relation type="other databases" type-id="38320e40-9f4a-3ae7-8cb2-3f3c9c5d856d">
<target id="919716e0-9cf7-410f-899e-d86ee0880f96">https://rateyourmusic.com/release/comp/abba/abba_gold__greatest_hits_f2/</target>
<direction>forward</direction>
</relation>
<relation type="other databases" type-id="38320e40-9f4a-3ae7-8cb2-3f3c9c5d856d">
<target id="fad038e8-2d44-4fc8-8e4d-0bfb4f5e7dd3">https://www.musik-sammler.de/album/35556/</target>
<direction>forward</direction>
</relation>
<relation type="other databases" type-id="38320e40-9f4a-3ae7-8cb2-3f3c9c5d856d">
<target id="ec7cc4e8-035f-4f65-a8d2-44533662e4bc">https://www.offiziellecharts.de/album-details-1499</target>
<direction>forward</direction>
</relation>
<relation type="review" type-id="c3ac9c3b-f546-4d15-873f-b294d2c1b708">
<target id="c904bedf-b21f-468d-a284-9efbdadea86b">http://hitparade.ch/showitem.asp?interpret=ABBA&titel=Gold+-+Greatest+Hits&cat=a</target>
<direction>forward</direction>
</relation>
<relation type="review" type-id="c3ac9c3b-f546-4d15-873f-b294d2c1b708">
<target id="77a2c0e5-d9c3-4ae4-99e0-b578ab415906">https://pitchfork.com/reviews/albums/abba-gold-greatest-hits/</target>
<direction>forward</direction>
</relation>
<relation type="wikidata" type-id="b988d08c-5d86-4a57-9557-c83b399e3580">
<target id="371bcad6-4194-4548-a0f8-e5dbdcfbf9e9">https://www.wikidata.org/wiki/Q286675</target>
<direction>forward</direction>
</relation>
</relation-list>
but neither of these are processed by Picard.
Writing a plugin to process these would not be difficult (and you can probably use an existing plugin as a base to tweak), but you would need to decide what metadata tags to put them into bearing in mind the very wide variety of types of URL that can be in a release record.
Also, bear in mind that Artists also have URLs that you might find useful / interesting and these would need a plugin (like my own Artist Official Website) plugin that makes extra API calls to get these.
Thanks for the reply. I have no experience with plugins, though. Is there a particular plugin I could start with to try and jerry rig?