You can't reference a pull request directly, but you can always get hold of a specific commit. You just need to find the commit hash that the pull request introduces. (If you're using github it's in the commits tab).
Then you'll need to use something like this in your composer.json -
"require": {
"mysoftware/thesoftware": "dev-master#3f38376d"
}
Where mysoftware/thesoftware
is the usual vendor/software name thing you would use, and the part after the '#' on the right-hand side is the specific commit hash you want.
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