I need to be able give users a link to my site with a parameter that will control their experience on the destination page, but, of course, Moqui does not allow parameters to be passed as a GET transaction. What are ways that I can work around that? It needs to be something that can be sent in an email, via sms and audibly.
An error message would be helpful know exactly what you are running into, but it sounds like the constraint to mitigate XSRF attacks.
The error message for this situation explains the issue and the recommended solution: "Cannot run screen transition with actions from non-secure request or with URL parameters for security reasons (they are not encrypted and need to be for data protection and source validation). Change the link this came from to be a form with hidden input fields instead."
You can pass URL parameters to screens to be used in code that prepares data for presentation, but not to transitions for code that processes input. The solution is to use a hidden form with a link or button to submit the form (that can be styled as a link or button or however you want). This is slightly more HTML than a plain hyperlink with URL parameters, but not a lot more and there are examples in various places in the Moqui itself.
If you are using an XML Screen/Form you can use the link element to do this with the @link-type attribute set to "hidden-form" or "hidden-form-link" (which just uses a hyperlink styled widget instead of a button styled one). If the @link-type attribute is set to "auto" (which is the default) it will use a hidden-form automatically if link goes to a transition with actions.
In plain HTML one possible approach looks something like this:
<button type="submit" form="UserGroupMemberList_removeLink_0">Remove</button>
<form method="post" action=".../EditUserGroups/removeGroup" name="UserGroupMemberList_removeLink_0">
<input type="hidden" name="partyId" value="EX_JOHN_DOE">
<input type="hidden" name="userGroupId" value="ADMIN">
</form>
Note that the button element refers to the form to submit, so can be placed anywhere in the HTML file and the form element can be placed at the end or anywhere that is out of the way (to avoid issues with nested forms which are not allowed in HTML).