It could be you can find s-parameters of your transistor in internet. However you can also find them in the data-sheet
https://www.sedi.co.jp/pdf/FHX76LP_ED1-2.pdf, page 4.
Select the table from the first line (that begins with frequency 1000) to the last line ending with angle of 146.1. Now copy with ctrl C. Then open Excel and paste with ctrl V.
In Excel select the first column then in the menu press data-->text to column... in the box choose "delimited" and then separator "space".
Now you will have the S-parameter divided into 9 columns. Insert a new row at the beginning with the text:
# MHZ S MA R 50
that is the string in the touchstone format that says: frequency in MHz, S-parameter, Magnitue and Angle type, Reference 50 ohm.
If you want you can insert one or more lines with comments: they have to start with !.
Now save as text tab delimited. Change now the file extension from .txt to .s2p
I did the work for you, just change the extension.
Now, in ADS select in the component palette "data items' and move "2-port S-parameter file" into your drawing, the brows the s2p file.
This allows you to perform the linear simulation under the conditions the s-parameters have been taken (i.e. 2V, 10 mA).