Add Custom Field to Generate Invoice

[Ember Robb]

I've been doing a lot of searching here and Sage Intacct's community, but haven't found anything for my use case.

I'm looking to add two custom fields (dates) to the "Generate Invoice" section for when a user creates an invoice. What I am trying to accomplish is having a user enter the date of the first and last charge in the invoice; I plan on using these dates for our integration. Basically, I use the dates to get timesheets and order entries that fall within that timeframe. I haven't found a better way to get order entries/timesheets associated with a posted invoice. (Although, I am open to suggestions)

Here is where I'd like to add my custom field to:

 

If anyone has any suggestions or can provide help, that would be appreciated.

Thanks