You can transfer the progress from your mobile device to Steam with the help of a Plarium ID:
1. Install the Plarium Play app from the official website plarium.com on your PC
2. Log into your current Plarium ID (a detailed guide on how to do that is here)
3. Tap on "Settings" in Plarium Play
4. Hit "Steam" in the Linked Accounts tab and enter your login credentials for Steam
5. Log into Steam using the linked account
Afterward, download RAID: Shadow Legends from Steam.
Please note that if you have already downloaded RAID from Steam before linking the existing account to it, you will start a new one.