For an existing advertiser account to be added to an external organisation, the organisation owner will ask you to provide your Advertiser Account ID. To find it:
Select the advertiser in the dropdown and click on the settings page in the navigation bar.
In the details tab scroll down to the very bottom to find your account ID. You can either click the copy icon and it will be saved to your clipboard.
Send the advertiser ID to the organisation admin and they are able to send an add request. When the advertiser ID has bene added the request will need to be approved on your settings page.