for an activation simulation of a cyclotron I want to include a source inside of it which simulates uniform particle loss around the circumference of said cyclotron.

Is this possible without adding a magnetic field in the simulation?

Yes that’s possible. You have to write and compile a source routine and I refer you to the manual , the training course slides or discussions on this forum if you have never done that.
Here the steps, assuming that the cyclotron orbit is in the x-y-plane:

Sample an angle phi uniformly from [0, 2 pi]. I assume the angle counts in mathematically positive sense from the x-axis.

If r is the radius of the cyclotron circumference, then your random starting position is
(x,y) = (r cos phi , r sin phi)

The tangential unit vector that you need for defining the particle direction in the source routine is
(x’,y’) = (-sin phi, cos phi)

On top of @totto reply, I would like to add the link to the lecture on the source routine from the last Fluka course. There, the new source_newgen.f routine is presented. It makes it easier for the user to prepare their own source. You will find several examples of its use in this forum.