Business Central API - Create Custom API Page for Sales Invoice Lines

I need to make a custom API Page on Dynamics 365 for Business Central. I need to get the AL source for the API Method.

https://api.businesscentral.dynamics.com/v2.0/{{TenantID}}/{{Environment}}/api/beta/companies({{CompanyId}})//salesInvoices({{InvoiceId}})/salesInvoiceLines

Does anyone know where the AL source for the SalesInvoiceLines method shown above can be found?

I copied the AL source from my Docker Windows Container to the directory C:bcSandboxapplicationsAPIV1Source Exclude APIV1 .Source.

I can only find the AL source for a ListPart for the SalesInvoiceLines (see below). I could be wrong, but I don't know how to use a ListPart to POST new records to BC.