I've asked this before but never had an answer.
I understand why it was sought after but I can't seem to find much information around what the UK had to do to get it.
From what I have read it seems clear that Germany made a concession for the UK, but in return for what? I did find something that suggested the UK was required to recognise newly formed Croatian and Slovenian states, but I can't seem to find anything substantial that backs this up.
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1. Where’s this idea found?
This conflation of UK (and EEC) recognition of Croatia and Slovenia with the Social Chapter opt-out negotiations can be found in a Counterpunch article from January 2017. OP, I noticed from searching AskHistorians that you asked this same question a year ago. Was that after reading this article, or had you seen it in other sources too?
Many - though not all - academic accounts of the negotiations (more later) consider these to be somewhat more separate issues.
The Counterpunch article lists in its references Misha Glenny’s The Fall of Yugoslavia: The Third Balkan War. Google books locates the following to p.192 of Glenny. (Unfortunately the book doesn’t have a preview, only search within, and no more of the page became visible on searching for words later in the extract.)
…Britain so many concessions at Maastricht, then it would be churlish to oppose Germany’s main foreign policy interest of the early 1990s, particularly as Britain’s interest in Yugoslavia was limited. The issue was then decided within the framework of some rather Machiavellian horse-trading. Bonn went full steam ahead with its…
(Perhaps if someone reading has a copy of the book, they may be able to add more.)
It’s probable that the writer took this line of reasoning from there. Misha Glenny was a 1980s-1990s Guardian and BBC correspondent for Central Europe, and covered the former Yugoslav wars for the BBC.
The Die Welt interview with Hans-Dietrich Genscher also referenced in Counterpunch does not mention Britain/UK or Maastricht at all.
Elsewhere on the web, there are a few old pages of questionable provenance which reproduce some text that also conflates the EMU, Social Chapter and Yugoslav recognition talks, and which quotes “Western European Union document 1342” dated November 1992. (The Western European Union was an international defence organisation which is now defunct and subsumed into the EU.) Some WEU documents can be found online, but it looks like this one is only available in the Historical Archives of the EU, so can’t be checked without going to Florence (assuming it's available at all and not covered by their 30 year rule). This looks like the earliest example of this text. A 1996 page mentions, in line with the heading of that article, “a recently published speech by T.W. "Bill" Carr, associate publisher of the London-based Defense & Foreign Affairs magazine. Carr gave this speech at a symposium on the Balkan war in Chicago on Aug. 31-Sept. 1.”
The quotation attributed to the WEU document states:
"The Maastrict [sic] process therefore influenced the Twelve´s reactions to the Yugoslav crisis, while that crises influenced the process of the European Union….it should be underlined that any undertaking that did not have the consent of all member states of the Community would have significantly weakened Europe just when it was negotiating the Maastrict Treaty and then proceeding to ratify it."