I will add to the suggestions you got,
If you have a tower near your airport, you can call them or, as when I was
doing my instrument training, we flew into and out of cedar city Ut. no
tower, so we called center and got our clearance from them.
I fly out of HND and VGT, when the tower is closed, I call LAS.

I looked up your airport, it looks like SEA is only about 7 miles from
you, I would call them for your clearance. File with FSS, then about 15
minutes before your ready to go, call them on the radio and tell them who
you are, where you are and what you want.

endre wrote:

I know this question has probably been answered thousand times before
on this forum. I am based at s50 in washington with no instrument
approach or departure procedure. It is class G to 1200 and then class
e plus some class b around.

How can I depart IFR from this airport? Do i need to depart VFR and
stay below 1200 and pick up my clearance or can I get a void time
clearance, be responsible for my own obstacle clearance and separation
and depart essentially at 0/0 (if i ever thought that was a good
idea?) The minimum vectoring altitude is 2000 feet. Just thinking
about those days where the ceiling is 800-1200 or a 400 feet fog layer
where it would be nice to get out.