How do you insert a record in a second form with a lookup field that is being created in the current form?

How do you insert a record in a second form with a lookup field that is being created in the current form?

I have two forms that are connected via a lookup field. When I successfully create a record in Form1, I want to add a record in Form2. 

My code looks something like this:

insert into Form2

[

LookupField = input.LookupField

<field> = <expression>

]

I get the following error (LookupField=200):

Error details:

Error in executing On Add - On Success script.

Due to invalid expressions insert statement is terminated Line:(12)

Error on inserting value for field 'LookupField'. Given value '200' doesn't exist in 'Form1' form Line:(16)

How do I work around this?