Silakan isi Pemberitahuan Pabean.

English Translation

Please fill out the Customs Declaration Form.

I don’t think “silakan” is the correct word in this context. According to several native speakers on HiNative, “silakan” is used when you kindly offer something to your guest etc.
Filling in a customs declaration form is an obligation even though staff may ask you to do so very politely. “Mohon” is the correct one, I guess.