Open Settings → Rules → New rule. Give it a Rule name, then choose one of three tabs:
Simple: words in the message. Type Words or phrases, separated by commas (for example dopłata, paczka), choose Must contain all of them or not, and Whole words only if you like. Then pick what happens: Block, Quarantine, Only warn or Mark as safe.
Sender: who sent it: These numbers or names, Numbers starting with, Numbers from countries (ISO codes, e.g. NG, CI), Any unknown sender or Any sender name instead of a number.
Advanced: combine conditions with All of these, Any of these and NOT: text, patterns, links (domains, short links, number of links), attachments, the sender's country, trust or type, the time of day, the risk score, and what other checks found. Under Then: the Decision, Add to risk score, blocking or trusting the sender, the Notification and Stop checking other rules after this one.
Before saving, try it with Test this rule (see "Testing a rule"), then tap Save. 1krBlocker may offer to re-check quarantined messages with the new rule.
Patterns in the Advanced tab are safe: a badly written one can't slow down or freeze the phone.