An employer posting a worker in the Czech Republic shall inform the relevant regional branch of the Czech Labour Office no later than on the day the employee is posted.
The information requirement can be fulfilled by sending the standard form (available here: https://www.mpsv.cz/documents/20142/3671566/Oznameni_vyslani_pracovnika.pdf/d266e68d-0645-135c-2dbb-079371cbd0e3) by post, data box or e-mail to the regional branch of the Czech Labour Office (for Prague, please see below).
Labour Office of the Czech Republic, Regional office for the capital city of Prague
Domažlická 1139/11
130 00 Praha – Žižkov
IČO: 724 96 991
call center: 950 178 111
electronic filing office: podatelna@aa.mpsv.cz
data box: w7aznwp
website: https://www.uradprace.cz/
During the period of posting, the employer shall keep a register of the posted employees at the workplace, as well as the relevant employment contracts (with a free translation in Czech).