Please see Note 578959.1 on  Populating GL_INTERFACE - Assigning Values to Optional Columns (i.e Reference Fields).

Example:

 ATTRIBUTE1 throughATTRIBUTE 10: Enter values for your descriptive flexfield
"Journals - Journal Entry Line". The values you enter depend on how you defined your
descriptive flexfield in the Descriptive Flexfield Segments form.

ATTRIBUTE11 throughATTRIBUTE 20: Enter values for your descriptive flexfield
"Journals - Captured Information". The values you enter depend on how you defined
your descriptive flexfield in the Descriptive Flexfield Segments form. The context for
Journals - Captured Information is the natural account value of the account used on each line.

CONTEXT: Enter the context field value for the descriptive flexfield "Journals - Journal Entry Line"
that identifies the structure of your descriptive flexfield. If you enter a value, you can also enter some
combination of values in the columns ATTRIBUTE1 through ATTRIBUTE10.

CONTEXT2: Enter Yes to identify your Value Added Tax Descriptive Flexfield structure.
You must use this column if you import data for the Value Added Tax Descriptive Flexfield.
Enter No to indicate that your journal entry line is not a tax item.
If you enter No, the four Value Added Tax Descriptive Flexfield related columns must be null.

CONTEXT3: Enter the context field value (natural account) for the descriptive flexfield
"Journals - Captured Information" that identifies the structure of your descriptive flexfield.
Enter a value only if you are importing the descriptive flexfield "Journals - Captured Information" without validation.
If you enter a value, you can also enter some combination of values in the columns ATTRIBUTE11 through ATTRIBUTE20.

Tjhank you.

You can do as above, but if you don't mind using the reference_1 field instead:

gl_interface.reference21 populates GL_JE_LINES.reference_1