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The short version is "you don't". You've bought a subscription (the level of which @Alex Koxaras _Relational_ has already explained).
There is, of course, a version of the software behind it, but, as @Barnali Putatunda 's linked question partially explains:
You can get the current version from the HTTP headers
The version format may change, or it may be removed completely
We can't tell you what the version was when you purchased
The version can, (and will) change, frequently, without warning, at any time
It's utterly useless to you, because you don't report problems against a version, it's of no use to auditing or compliance, and you can't stop it changing.
A bit curious why you need the Trello version. For the Atlassian cloud products, generally no specific version is exposed to end user.
However, I find an old question in the community with the same question & that might help you.