English equivalents, depending on which national and regional dialect you speak, could be "to call shotgun" (although this is very specific, referring to sitting in the front passenger seat of a car), "to call dibs" or "to bagsy". You can use pax with the verbs få or ha, but can also use the verb paxa on its own, for example jag fick pax på cykeln or jag har paxat cykeln (I called dibs on the bike). This probably comes from the Latin word pax meaning "peace", and it's likely that the phrase pax för in the sense of "dibs" and the verb paxa come from the meaning of "safe space".