SQL How to Convert Single Tenant Databases to a Multitenant Database
sql-server-2016
This can be made to work using SESSION_CONTEXT (new in SQL Server 2016). In previous versions CONTEXT_INFO could be used, but it is less robust (cannot be read-only, only one slot per session).
Example
First, set a DEFAULT on the underlying table's CustomerId column:
CREATE TABLE ReportingDB.[dbo].[CustomerTransaction]
(
    [CustomerTransactionid] [int] identity(1,1) primary key NOT NULL,
    [Customerid] [int] NOT NULL
        DEFAULT TRY_CONVERT(integer, SESSION_CONTEXT(N'CustomerID')),
    [QuantityBought] [int] NULL,
);

Then define the view as follows:
CREATE OR ALTER VIEW dbo.CustomerOneTransaction
AS
    SELECT
        CT.CustomerTransactionid,
        CT.Customerid,
        CT.QuantityBought
    FROM ReportingDB.dbo.CustomerTransaction AS CT
    WHERE
        CT.Customerid = 1
        AND TRY_CONVERT(integer, SESSION_CONTEXT(N'CustomerID')) = 1
    WITH CHECK OPTION;

Each session would need to set the CustomerID key once using sys.sp_set_session_context:
EXECUTE sys.sp_set_session_context
    @key = N'CustomerID',
    @value = 1;

If the key should be immutable for the life of the connection (typically true), use the @read_only option:
EXECUTE sys.sp_set_session_context
    @key = N'CustomerID',
    @value = 1,
    @read_only = 1;

This arrangement allows your insert query to work, and also ensures that CustomerID always matches the session key when the view is used to insert or update, otherwise a CHECK_OPTION error is returned. For deletion, illegal operations are ignored rather than raising an error.
This answer addresses the question asked, but in practice I would almost always prefer a database per customer, for security, manageability, and availability reasons.
Demo: db&lt;&gt;fiddle
Additional Information
You can also use this pattern with Row-Level Security to enforce customer isolation without views. Database-per-customer is the pattern used most for Software-as-a-Service companies (not banks and finance companies - the benefits of customer database isolation don't really make any sense for many other industries).

Multi-tenant applications with elastic database tools and row-level security - Microsoft Docs
SQL Server Databases in the Cloud – Single Tenant or Multi Tenant? - Dr Greg Low
Row Level Security Limitations, Performance and Troubleshooting - Aaron Bertrand