Current State:

When using Add Service Space Customer automation, there will be a delay on when the customer will be displayed in the project.
For Premium/Enterprise customer , they can add "Delay" action to wait until the customer available in the space , then perform edit work item action to update a user picker field , such as Request participant.
But, for Standard customer , there is no way to add the Delay.

Proposed Future State:

The action performs internal checks if the customer already exists in the space before ended the action. For example use this API Get Customers
 

Alternative (e.g. Third Party Plugin):

No workaround