Get Marketing Cloud UserID (numeric)

I'm currently following this guide:

https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-programmatic-content.meta/mc-programmatic-content/createDataExtension.htm

To create a data extension via a landing page.

It works, but I'm stuck at the part where you have to provide ClientID to add data retention settings to the DE. You have to provide ClientID and UserID:

SET @client = CreateObject("ClientID")SetObjectProperty(@client, "ID", @memberID)SetObjectProperty(@client, "UserID", @userIDwithPermission)

The guide says I can get the UserID by hovering over a name in the Users list, but there seems to be no such option. I get a very long string, instead of a numeric UserID. I tried using the Username but it returns this error:

Error Message: Object of type 'System.String' cannot be converted to type 'System.Int32'.

So clearly I'm looking for a number rather than a string. How on earth do I find UserID?