Sets the proposal's customer.

POST {{baseUrl}}/proposals/:proposal-id/customer

This endpoint sets the proposal's associated customer and returns the new state.

{"registrationNumber"=>"<string>", "sourceCountryCode"=>"<string>", "companyType"=>"<string>"}

Key	Datatype	Description
`If-Match`	string	(Required) The If-Match HTTP request header makes the request conditional. For PUT and other modifying methods, it will only modify the resource in case the ETag indicated by the If-Match header is identical to the resource's current ETag.
`Content-Type`	string
`Accept`	string