Specify the ALLOY parameter with a value 39. See section 20A.4 of the International codes manual.
Note that this parameter is meant solely for design per the American Aluminum code in STAAD. If does not have any effect on the analysis results (member forces, joint displacements, support reactions, etc.). The alloy is used by STAAD exclusively to find the allowable stresses that will be used to check the adequacy of the member for interaction of axial and bending effects during aluminum design.
If you are interested in only the analysis, just specify the pipe diameter and wall thickness under the member property block of input. And, for material constants, specify aluminum as the material. If you are looking for a specific E which is different from the built-in value for aluminum, create a material with those values and assign it.