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The REST API below use to retrieve a list of indirect resellers with whom the specified customer has a relationship. If the customer do not have the indirect resellers that have a relationship, it will return null using this API. Please check it again.
{baseURL}/v1/customers/{customer-id}/relationships
Best Regards,
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Updated:
Invite a customer to establish a reseller relationship with you, please check the steps below:
1.Select Customers from your Partner Center menu and then select Request a reseller relationship.
2.On the next page, review the draft email message. You can open the draft message in email or you can copy the message to your clipboard and paste it into an email.
More information is here:
https://learn.microsoft.com/en-us/partner-center/indirect-reseller-tasks-in-partner-center
We can use the REST API below to retrieve a list of the signed-in partner's indirect resellers.
{baseURL}/v1/relationships?relationship_type=IsIndirectCloudSolutionProviderOf
https://learn.microsoft.com/en-us/partner-center/develop/retrieve-a-list-of-indirect-resellers