<div class="abstract">
                        <p>Sets the unique identifier for the user.</p>

<p>Setting a user ID ensures subsequent Merchant activity (such as installs or purchases) is attributed to a user.
This can either be your user ID, email or a stable hash of one. You can use this later to look up orders, activity and identify the user in Webhooks.
String with a maximum length of 255.</p>

<p>@note: You should set the user ID in your login handler.</p>

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