There is no RAW answer. It must be decided by the DM.

Firstly, this ambiguity is likely an intentional, direct consequence of 5e's design ethos of "rulings over rules" and the prominent role of the DM as the person that must make those rulings in the case where the rules are silent. Illusions and illusion magic are one area in which there is essential no general rules guidance, and thus falls squarely into the domain of DM-rule.

Jeremy Crawford said as much when asked about this exact question on Twitter:

Q: Can a creature under the effects of Phantasmal Force be restrained by it, i.e chains wrapping around a creature?

A: Phantasmal force details the few game effects that it can reliably create. A DM is free to allow additional effects.

Because illusions are a complicated issue, I will outline two of the major DM approaches to this issue. Since they take opposite stances to each other, any DM reading this will have to figure out which way makes the most sense, seems the most fun. and/or which way aligns better with how they've already been handling it.

The core of the disagreement is whether the DM rules that an illusion fools the mind so thoroughly that the body will react physically to it or not.

Interpretation 1: No, Phantasmal Force cannot chain someone and force them prone

In this argument, the spell looks and feels like a chain, but the chains (not being real chains) would not be able to restrain the creature since they would offer no resistance when the creature tries to struggle out of them.

The target rationalizes any illogical outcomes from interacting with the phantasm

Because of the above effect from the spell, the mind of the creature would try to rationalize this in some way (the chains broke, the chains loosened temporarily, etc.) but they would still believe the illusion to be true.

In this way the creature would not gain the prone condition because their movement is not actually inhibited in any way even though they believe the illusion is real.

See this answer for a more thorough take on this interpretation.

Interpretation 2: Yes, Phantasmal Force can chain someone and force them prone

In this interpretation, the chains really do restrain the creature because the creature treats the chains as real chains, even though they are not.

While a target is affected by the spell, the target treats the phantasm as if it were real.

The chains also sound and feel real to the target in every way.

An affected target is so convinced of the phantasm’s reality that it can even take damage from the illusion.

So, in this case, when the creature tries to escape from its restraints, its mind, thoroughly convinced that the chains are real, will act under that assumption. Thus, the creature's limbs will be bound. And this creates the case for the fact that the creature is able to gain the restrained condition from this spell.

See this answer for a more thorough take on this interpretation.

Decide but stay consistent

So I have provided very brief overviews of two interpretations of the spell's effect. There are certainly more of them out there. As a DM, the key is to decide on one and apply it fairly, transparently, and consistently. Not only across this spell but other similar ones.

You could certainly Dash first to increase you movement allowance, then attack the first mook, then move, then attack the second mook. That's not exactly what you're asking about though, since you want to see the result of the first attack before deciding to Dash.

In the strict rules as written, however, only movement is explicitly allowed to be broken up arbitrarily. Taking the Attack action doesn't "add to your attack allowance" the way Dash adds to your movement allowance.

Implicitly, when you take an action, you are committed to completing the entire action unless you have a rule that says otherwise. "Moving Between Attacks" is one such rule, and object interaction (PH p.190) is another:

You can also interact with one object or feature of the environment for free, during either your move or your action.

By this reading, you have to complete all the Extra Attacks that are part of the initial Attack action before using Action Surge to Dash.