In your custom save action (the microflow) you’ll have the object as the input parameter. This includes all the fields and the references, so the values that you selected in the dropdowns.
Let’s say the object that you are saving is called Order. This will be your input parameter for the microflow. Then based on this input create the a new object (containing the auto increment attribute) by adding a create object activity and map the data from the input object to the attributes or references of this object. For example you have a reference between the Order object and entity H and the new object created has the same type of reference then in the mapping map the association to $Order/Order_H. If the new object has attributes to hold the data you’ll need a retrieve action for the object H. So then the microflow would first have a retrieve action, getting the H object by association from the Order object. The next action would be the create object action in which you map the attribute of the new object to $H/fieldname.
Hope this helps in finding a solution.