Today we see how to make custom API in business central.
I ma making API for Item data.
Step 1: 
add new file in project and set the any name.al Write the below code in the file.
page 50103 Products
{
    PageType = Card;
    SourceTable = Item;
    layout
    {
        area(Content)
        {
            group(GroupName)
            {
                field("No."; Rec."No.") { }
                field(Description; Rec.Description) { }
                field(Blocked; Rec.Blocked) { }
                field(Type; Rec.Type) { }
                field("Base Unit of Measure"; Rec."Base Unit of Measure") { }
                field("Last Date Modified"; Rec."Last Date Modified") { }
            }
        }
    }
}
Step 2: Add new file in the project and set the name.xml. In file write twebservices and click enter you will automatically get the structure.
<?xml version="1.0" encoding="UTF-8"?>
<ExportedData>
    <TenantWebServiceCollection>
        <TenantWebService>
            <ObjectType>Page</ObjectType>
            <ServiceName>Products</ServiceName>
            <ObjectID>50103</ObjectID>
            <Published>true</Published>
        </TenantWebService>
    </TenantWebServiceCollection> 
</ExportedData>
Step 3: Now run the project it automatically publish your api.
Step 4: Search web service in business central search bar and click on it.
Step 5: You can see the webservice in the below image.
In the next post we will look at how we use our API on postman.