I am not much proficient with ASCX code. That's how I think it can be done. But first of all why ASCX? why not Portal Engine?
Below is the reference link to the BizForm Class/events/properties. You should be able to find your answer here
- On BizForm load first get the data for a particular id from a CT using CustomTablesItemProvider.
- Do type casting and error handling for the data you receive from CT.
- On OnLoad method of BizForm access the field you want to pre-populate and assign the value.
This should do the trick.
p.s :- You can also see the solution provided by Rocky here. Rocky is Kentico's developer advocate - https://stackoverflow.com/questions/29668621/how-to-sign-value-to-a-field-in-bizform-in-kentico-8-2