There is no direct option or settings you can select to use a fully developed flow in the inlet. Having said this, since you are solving a laminar flow, you can write velocity as a function of radius of the pipe.
To do this, you will have to use a UDF. A UDF is a program which can be compiled, loaded, and hooked to Fluent. You might have to use the DEFINE_PROFILE macro. Here is an example from the Fluent Customization manual you can use almost readily. Please make sure you modify the equation to the fully developed flow velocity.
Here is a video (by Raef Kobeissi) which talks a bit more about using UDFs.
I hope this helps.
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