Hi @Glenn Skinner ,
To help you better, do you want to set a specified email address where you wish to send access request? If my understanding is wrong, please contact me in time.
If yes, you can use the ** SPWeb.RequestAccessEmail
** property, the property can set the e-mail address to which requests for access are sent. Here is an example in the following document, you can refer to the code in this document: SharePoint : Update Access Request Settings email address using SSOM
More information for reference:
SPWeb.RequestAccessEmail property
Set up and manage access requests
Hope it can help you. Thanks for your understanding and support.
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