is it possible via script to have the player manually entering a text and then to dispatch it using the preselected UI in the dialog manager?
example would be:
A conversion start with anode from an NPC saying something
the player answer manually using an inputfield prefab independent from your dialog system
a script inject the player entry in the preselected dialog manager UI like a player node does in a conversation.
Yes. This is what the built-in TextInput() sequencer command does. It shows an input field. When the player submits the input, it stores the value in a Dialogue System variable and the conversation continues. To see it in action, play DemoScene1 and interact with the computer terminal.
If you don't want to use TextInput(), you can write a custom sequencer command.