What is this part on the Me 262?
They are Werfer-Granate 21 rocket launchers -

The Werfer-Granate 21 rocket launcher, also known as the BR 21 (the
&quot;BR&quot; standing for Bordrakete) in official Luftwaffe manuals, was a
weapon used by the German Luftwaffe during World War II and was the
first on-board rocket placed into service by the Luftwaffe, first
introduced in mid 1943.

On the Me262 they were mounted one each side of the nosegear well.
This is the exact aircraft your model is based on -


Messerschmitt Me262A-1a 'Green 3' of the Geschwaderstab [headquarters unit] of JG 7 prepared
to move off across the concrete surface at Brandenburg-Briest in
February or early March 1945. The aircraft, finished in a relatively
rare application of streaked horizontal lines, has been fitted with a
pair of 21cm WGr air-to-air mortar tubes visible beneath the fuselage
aft of the nose wheel. (source)