You could accomplish that using either Zapier or the Flowsana integration I’ve created. Both support applying a specified tag to a task when the task is added to a particular project.
Each has its advantages and disadvantages. For example, at present Flowsana doesn’t support handing subtasks while Zapier does (though I’ll likely add that capability in the future). In contrast, Flowsana knows about and can work with custom fields, while Zapier doesn’t know anything about custom fields.
In my (biased) opinion, it’s much easier to create a rule in Flowsana than it is in Zapier; one of my design goals for Flowsana was to make it quicker and simpler to build a rule than Zapier requires. 
EDIT:
Since I’m discussing a product comparison, I realized I should also mention that while If-Then rules are the sole thing that Zapier does, If-Then rules are only one element of Flowsana’s automation capabilities.