Just use the automatic action Add task. You could also consider using the Reinitialise action which allows you to jump a request process to another one at the same status. With this method you have a main or stub request process which does all the common things and then when you want to invoke a particular request process, you 'reinitialise' and that workflow takes over.
I tried with the automatic action Add Task, but I didn't find a way to dynamically select which task. I was hoping that there was a simpler/built in way to do this without using Reinitialise. But I guess I'll have to go down that route for now.
One way of doing it is to create your attributes which drive your tasks on your Service object which then drive the process. You would then need to create decisions based on the attributes off the item you are requesting.
An example condition would be
Configuration Item Requested
If the value is true, add your automatic task action. If it is false, move on to the next condition or next step in the process.
The advantage of this method is that you can have different tasks fire off for different items you are requesting. The downside is, each time you want to create an additional dynamic task, you need to create new attributes, decisions and updates to the process. You want to keep the maintenance of the process to an acceptable level.
Hope that helps and makes sense.
Auto re-initialise didn't work either. It won't save anything that I select as runtime value for default process.
It does work, but you may need to persevere as sometime LD don't always give easy to understand error messages.. Reinitialise is an alternative to using tasks when what you might want to do is switch to a different complete process. I wasn't sure of exactly what you were trying to do, so I thought I'd throw it in there as a feature to check out. If you want to create tasks dynamically, then you can ask people to manually do an add task, but I'm also thinking you want to control this. A good example might be if the category is x then create these tasks. That is very commonly done and you'd have a decision box which then runs a few automatic add task actions.