Dear @Tareq.AbuAgolah,
I’m assuming you are referring to the fixed-bottom MHK RM1 r-test: r-test/glue-codes/openfast/MHK_RM1_Fixed at main · OpenFAST/r-test · GitHub; is that correct?
You can model a flexible substructure or a spring between the tower and ground in OpenFAST through the SubDyn and/or External Platform MCKF sub-structure modules (CompSub = 1 or 2). If you simply want a spring/stiffness matrix between the tower and ground, the later would be appropriate.
You can certainly output the tower deflection, using ElastoDyn outputs TwHt1TDxt or TwHt1TPxi, etc. for each tower output node. For an explanation of the difference between these two outputs, see my post dated Jun 13, 2017 in the following forum topic: SubDyn problem with larger substructures.
Best regards,