wookinguibento ni taikensankasuru.

You are right: normal Japanese is usually written without spaces.

So the standard written form is:

ウォーキングイベントに体験参加する。

Spaces are often added in teaching materials to help beginners see the parts more clearly.

Because it is made from loanwords:

• ウォーキング = walking
• イベント = event

Katakana is commonly used for words borrowed from other languages, especially English. So ウォーキングイベント is basically a Japanese-style borrowed expression meaning a walking event.

Here, に marks the target of participation.

With 参加する (to participate), the thing you participate in is usually marked with に:

• イベントに参加する = participate in an event
• 会議に参加する = participate in a meeting
• 大会に参加する = participate in a tournament

So in this sentence, ウォーキングイベントに means in the walking event or to the walking event in the sense of joining it.

参加 is a verbal noun. Many Japanese nouns can become verbs when combined with する.

So:

• 参加 = participation
• 参加する = to participate

This is a very common pattern:

• 勉強する = to study
• 運動する = to exercise
• 参加する = to participate

You may sometimes see 参加をする, but 参加する is the basic and more natural form in most cases.

In this sentence, 体験参加 is best understood as a single compound noun, and then する turns it into a verb.

So:

• 体験 = experience / trial
• 参加 = participation
• 体験参加 = trial participation / joining as a trial
• 体験参加する = to participate on a trial basis

This does not usually mean doing two separate actions, experience and participate. It means joining something temporarily or as a trial.

A natural English explanation would be something like:

• to join on a trial basis
• to try participating
• to attend as a trial participant

Yes, it is understandable and often natural in the right context, especially for things like:

• clubs
• lessons
• community activities
• organized programs
• events people may try once before joining fully

It has a slightly promotional, informational, or practical feel, like something you might see in an announcement, flyer, website, or application form.

In casual speech, people might also say something simpler, depending on context, such as:

• 試しに参加する = participate just to try it
• 一度参加してみる = try participating once

So 体験参加する is natural, but it sounds a bit more like a set phrase than everyday casual conversation.

Japanese often omits the subject when it is clear from context.

So this sentence could imply things like:

• I will participate in the walking event on a trial basis
• We will join the walking event as a trial
• [Someone] participates in the walking event as a trial

Japanese does this very often. If needed, a subject could be added:

• 私はウォーキングイベントに体験参加する。 = I will participate in the walking event on a trial basis.

But in many situations, adding 私は would be unnecessary.

する is the plain non-past form.

In Japanese, the non-past can mean:

• present, depending on context
• future, depending on context
• habitual action, depending on context

So ウォーキングイベントに体験参加する。 could mean:

• I participate in the walking event as a trial
• I will participate in the walking event as a trial

Context decides which one is intended.

Also, because it is in the plain form, it can sound like:

• a neutral statement
• a note in a schedule or plan
• a headline-style sentence

The polite version would be:

ウォーキングイベントに体験参加します。

It can be a complete sentence, but in plain form like this, it can also sound a bit like:

• a plan
• a memo
• a schedule entry
• a heading or short description

For example, on a website or flyer, plain non-past forms are common in short explanatory text.

In everyday conversation, you would often hear the polite version instead:

ウォーキングイベントに体験参加します。

That sounds more like a normal spoken sentence.

Normally, with 参加する, the standard particle is に:

• イベントに参加する

Using へ here is generally not natural.

Using を directly with the event is also not the normal pattern. So:

• natural: イベントに参加する
• unnatural/basic-grammar mistake: イベントを参加する

You may see 参加をする, but that is different: there, 参加 itself is the noun being used with する. The event you are joining is still marked with に:

• イベントに参加をする

Even so, イベントに参加する is the most standard and natural form.