Azure B2C - Pre create federated user using Graph API

Question

Hi, I would like to create "federated" type users using graph api in advance so that when the external SSO users try to login they get matched with existing profile instead of creating a new one and also stops the unauthorised users to login.
I tried using graph api and user is gets created however I cannot login thru SSO (External IDP).

"identities": [
{
"Issuer": "XYZ.COM",
			"IssuerAssignedId": "******@xyz.com",
			"SignInType": "federated",
			"AdditionalData": null,
			"ODataType": null
}]

I can see some other forums where people suggests that the "issuerAssignedId" needs to be unique id (oid) from the external IDP which I don't have access to. My question is how can I link the external user with this pre created account using the "email address" field which is getting passed as claim token from external IDP? Thanks in advance Abdullah