Specify default values for custom fields on an ideas form based on the URL.

We have a requirement to allow some users to click on a link to an ideas form (within a submit only portal) which pre-populates certain custom field values.

The reason for this is to make the user experience better instead of needing them to select from a drop-down list with lots of values. Different users would then use the same form but with different pre-populated content.

We found a way to do this with the ideas category field using this type of link https://xxxxxx.ideas.aha.io/ideas/new?ideas_idea[idea_category_ids]=6847463579149137106 and would like to do a similar thing but using custom field values (pre-defined list custom field)