Setting page property via drop down in a user macro

[Max E]

Hi there! 

I am trying to set a page property by using a user macro. I tested several approaches shown here in the forum but none did work.

The case: This drop down should set the page property that will be used in K15 pdf exporter.

## Macro title: My tiny title
## Macro has a body: N
## Output: String so far
##
## Developed by: me
## Date created:01/22/2018
## Installed by: me
## Copyright: none
## @param Doctype:title=Doctype|type=enum|desc=Please choose.|enumValues=Option 1,Option 2, Option 3|required=true

$paramDoctype

Now I want to set the $paramDoctype as page property "DocumentType". 

Is that possible by using a user macro? How?