How to populate quota amount field on forecasts tab using apex in salesforce?

Setting Quotas for Customizable Forecasting
Users with the appropriate permission can set their individual quota. Salesforce admins and users with the appropriate permission can edit any quota. And managers can change the quotas of users who report directly to them in the role hierarchy.

REQUIRED EDITIONS AND USER PERMISSIONS
Available in: Salesforce Classic
Available in: Professional, Enterprise, Performance, Unlimited, and Developer Editions

USER PERMISSIONS NEEDED
To set your quota:
Edit Personal Quota
To set any user’s quota:
Manage Internal Users

AND

Modify All Data
Do one of the following:To set your personal quota, access your personal information in your personal settings.
To change the quota of a user that reports to you, from Setup, enter Users in the Quick Find box, then select Users, and click the full name of the appropriate user.
Click Edit in the Quotas related list.
Select the appropriate range start and range length. If available, select a territory and product family.
NOTE If your organization uses custom fiscal years, you cannot set quotas for a period that does not have a defined fiscal year.
Enter the quota amounts for revenue and/or quantity.
Select the appropriate currency if you work with more than one currency.
Click Save to save the quota and return to the user detail page, or click Quick Save to save the values and continue to edit the quota.
TIP Using Quick Save allows you to quickly set a different quota for each product family.

NOTE If these options are not available to you, you may not have customizable forecasting. To determine if your organization has enabled customizable forecasting, see How Do I Make Sure That Customizable Forecasting Is Enabled?. Delegated administrators won’t see the quotas related list on user records.
You can also use tools like the Data Loader to do mass updates on quotas. When updating quotas, set the quota field value on the RevenueForecast or QuantityForecast objects.