sankamousikomi ni syuugouzikan wo kaku.

Here, に marks the place where something is written.

With 書く, Japanese often uses this pattern:

place + に + thing written + を + 書く

So:

• 参加申込に = on/in the application
• 集合時間を書く = write the meeting time

In English we say write the meeting time on the application, and Japanese uses に to express that on/in idea.

Because 集合時間 is the thing being written.

The particle を marks the direct object of the verb. Since 書く means to write, the thing you write takes を.

So:

• 集合時間を = the meeting time as the thing being written

A very literal breakdown is:

• 参加申込に = on the application
• 集合時間を = the meeting time
• 書く = write

So, Write the meeting time on the application.

参加申込 means something like application to participate or sign-up/application for participation.

Depending on context, it may refer to:

• the act of applying to participate, or
• an application form

In a sentence like this, it most naturally feels like the application form or the sign-up form.

In everyday Japanese, if you want to make form extra explicit, you may also see:

• 参加申込書 = participation application form / registration form

So this sentence may be a shortened way of saying something like 参加申込書に集合時間を書く.

集合時間 means the time everyone is supposed to gather/meet.

• 集合 = gathering, assembly
• 時間 = time

So it is not just any time; it is specifically the meeting time or assembly time.

For example, if an event starts later but everyone must arrive by 8:30, then 8:30 could be the 集合時間.

Grammatically, 書く is the plain dictionary form and literally means to write.

But in short instructions, notes, labels, and manuals, Japanese often uses the plain form in a way that feels like an instruction in English. So depending on context, 書く can be understood as:

• write
• write it
• you should write
• to write

If this appears in a list of instructions, it is very natural to understand it as Write the meeting time on the application.

Japanese often leaves out the subject when it is obvious from context.

So this sentence does not explicitly say:

• I write
• you write
• someone writes

In actual use, the subject is inferred. If this is an instruction, the understood subject is probably you.

That is very normal in Japanese. Pronouns are often omitted when they are not needed.

Yes. English usually says:

Write the meeting time on the application.

Japanese often puts the location before the object, like this:

参加申込に 集合時間を 書く

But because Japanese uses particles, the order is more flexible than in English. The particles tell you what each part is doing.

Here:

• 参加申込に = where to write
• 集合時間を = what to write
• 書く = the action

You may also see:

集合時間を参加申込に書く

That means the same thing. The particles keep the meaning clear.

Normally, に is the natural choice here.

With 書く, に marks the surface, place, or target where the writing goes:

• 紙に書く = write on paper
• ノートに書く = write in a notebook
• 申込書に書く = write on the application form

で usually marks the place where an action happens in a broader sense, but with writing, when you mean the actual place something is written on, に is the standard particle.

So 参加申込に書く is the expected phrasing.

It is understandable, but by itself it can feel a little like a memo, instruction fragment, or textbook example rather than a full everyday sentence.

That is because:

1. the subject is omitted,
2. the verb is plain 書く, and
3. 参加申込 may be a shortened expression for a form or application document.

In real-life Japanese, you might also see fuller versions such as:

• 参加申込書に集合時間を書いてください。
  Please write the meeting time on the application form.

• 参加申込書に集合時間を書く。
  A plain statement or written instruction.

So the original sentence is fine as a grammar example and works especially well as an instruction-style phrase.

Japanese does not have prepositions like English on, in, or at. Instead, it uses particles such as:

• に
• で
• を
• へ
• と

These particles show the role of each noun in the sentence.

So the English idea on the application is expressed with 参加申込に. It is not a one-to-one translation of on; it is just the natural Japanese way to mark the place where the writing appears.

Yes. It is a compound noun.

It is made of:

• 参加 = participation
• 申込 = application / sign-up

Together, 参加申込 means application for participation or registration.

Japanese often combines nouns this way, and the whole compound functions as one noun before the particle に.