You can proceed in one of two ways:
1. If you do not care about the actual number of viruses and just want a broad sense of presence and density, you can use a pipette to carefully drop your sample onto the anodisc while the vacuum is running.
This has the advantage of letting you put multiple samples on a single disc if you pipette carefully within an area. You can also use a PAP pen to divide the disc into sections (e.g. Vector Laboratories ImmEdge Hydrophobic Barrier PAP Pen #H-4000).
2. If you do care about being quantitative, attach a glass reservoir to the top of the vacuum flask with a pair of clamps. The order, from the bottom, is thus: Filter holder, 5µm backing filter, 0.02µm Anodisc, and glass reservoir. You can then pour your sample into the reservoir and allow it to evenly spread across the anodisc before turning on the vacuum.
This will allow you to perform accurate VLP counts.